In the last years there has been an increased attention given in the literature to ethical issues in business. Within academic and industry literature, codes of ethics are generally understood to act as a mechanism guiding and ensuring ethical behaviour. However, this premise has not yet been thoroughly explored. This study proposes to address this gap, investigating codes of ethics' use and effects on ethical behaviour. This research examines the tools used in ethical decision-making by New Zealand marketing research practitioners. The focus of this research is on client relationships. The study uses literature on business ethics, marketing research ethics, models of ethical decision-making, codes of ethics' use and effectiveness, trust in business relationships, reputation, and individual values and personal ethics. A conceptual framework evolved from the literature review and helped guide the research questions. The model drawn from the literature uses a three level approach: industry/organisational level, individual level and relationships level. The model indicates that at the industry/organisational level, three main dimensions may have an impact on ethical decision-making within client relationships: codes of ethics, size of organisation and affiliation. At the individual level, personal values, ethical perceptions and judgements are seen to guide individual behaviour. At the relationships level, trusting business relationships are believed to lead to a good and ethical climate between marketing research practitioners and their clients; at this level of analysis, dimensions such as trust, credibility and reputation form the basis of such relationships. Mixed research methods were used in the study. A preliminary survey was employed in order to provide background industry information which assisted with and informed the development of the structure of interviews with New Zealand marketing research practitioners. Accordingly, a qualitative research approach underpinned the thesis. Twenty-nine interviews were conducted in both the North and South Island. The participants were selected from small, medium and large marketing research organisations as well as members and non-members of Market Research Society New Zealand (MRSNZ). They reported on their awareness and familiarity with professional and organisational codes of ethics as well as on codes’ use. In particular, information on ways of dealing with ethical issues when they arise in their relationships with clients has been provided by participants. The study showed that there is limited use of codes of ethics by New Zealand marketing research practitioners albeit they identify the code as a hygiene factor which can guide ethical behaviour. Consultation with peers is a common way of dealing with ethical issues in client relationships. The findings also revealed that personal values and ethics are the tools that ultimately shape their behaviour. At the relationships level, trust and its adjacent constructs, credibility and reputation have a significant role in generating good and ethical business relationships. Following these findings, the model was revised accordingly. The findings and model proposed in this study contribute to literature and theory by providing an additional dimension for the analysis of ethical behaviour in client relationships. The actual findings on ethical decision-making of New Zealand marketing research practitioners have also implications for the practice of marketing research.