Although it is well known that users benefit from being involved in the implementation of information systems, it is an open question how trust and justice interact to achieve this in cases where the users have no say about the new IS. To our knowledge, this study is the first to quantitatively examine the relationships between perceptions of justice, trust and user motivation in the context of a knowledge management system (KMS). Specifically, we studied the impact of justice and trust in a KMS recently implemented at all 78 branches of The National Insurance Institute of Israel (TNIII). The KMS encompasses all aspects of TNIII activity, and enables fast, reliable and secure communication between TNIII employees and the public they serve. The sample consisted of 300 full time employees (response rate 62.6%). The respondents were randomly selected as potential survey respondents from 22 branches of TNIII. The researchers collected data over a four month period, traveling country-wide between the various branches. It was hypothesized that KMS users would more motivated to be involved when they feel they will receive their share of benefits from the KMS (distributive justice), are treated with fairness both formally and informally (procedural and interactional justice respectively), and when they are given enough information to make their involvement meaningful (informational justice). The findings from 190 completed questionnaires show that interactional justice affects motivation. Distributive justice increased trust, and lack informational justice decreased trust. However, contrary to predictions, neither procedural justice nor interactional justice significantly affected trust. Trust affected motivation to be involved. Overall, our study provides a starting point for a better understanding of how perceptions of justice and trust in the vendor of the KMS during the implementation process affect user motivation. The results show that some aspects of justice build trust while others build motivation. More specifically, trust in the vendor develops through distributive justice and informational justice as applied by the organization to the users. By identifying the rules individuals use to evaluate organization and supervisor fairness, managers can better grasp employees' justice perceptions and indirectly influence important organizational outcomes, including motivation regarding new KMS for leveraging organizational advantage. The perception of fairness is crucial in many organizational settings and should be considered in the context of IS implementation.