People effectiveness has become a key differentiator of competitive advantage in the knowledge economy and the need for a valid and reliable measure of people effectiveneness has become paramount for success. The research positions the psychological work immersion scale (PWIS) as a relevant measure of people effectiveness and explores the relationship between the PWIS variables (psychological attachment and people effectiveness enablers) and perceptions of business performance. Furthermore the research explores whether higher levels of psychological work immersion leads to increased business performance over time in an attempt to position the value of organisational development interventions aimed at increasing psychological work immersion levels in the work place. The setting for this research was a not-for-profit organisation in South Africa. The sample for the study was measured at two defined points in time over a 14-month period and consisted of n = 414 (T1) and n = 551 (T2). The study showed that the PWIS factor structure is a valid measure of the psychological work immersion construct across time (T1 and T2). The results provided evidence of convergent, intra-discriminant and external discriminant validity (construct validity) of the PWIS within (T1 and T2) and over time (T1 vs T2). The results showed that the PWIS has acceptable internal consistency reliability within and across time (T1 and T2) as well as demonstrating test-retest reliability across time. The results provided evidence that the people effectiveness enablers and psychological attachment variables significantly predict perception of business performance indicators (profit/loss, costs, and cash flow related to operating activities), and that strong perceptions of people effectiveness enablers relate to strong individual perceptions of business performance through a high sense of psychological attachment. The mediation results confirmed the test-retest reliability and validity of the PWIS in predicting perceptions of business performance within and over time. This finding shows that psychological attachment is an important factor in terms of iv influencing the individual perceptions of business performance which is related to improvements in actual business performance. The study also showed evidence of a positive relationship between psychological work immersion and business performance and demonstrated improvements in psychological work immersion coincided with year on year improvements in business performance. The study contributes towards the current literature on organisational development and specifically on the measurement of people effectiveness within knowledge economy organisations. / Industrial and Organisational Psychology / D. Com.