Knowledge intensive business services (KIBS) play an increasing role in business and economics as they serve as service providers for organizations outsourcing their knowledge intensive business processes (KIBP). Our literature analysis shows that KIBP as outsourcing objects remain considered as "black boxes". Based on the assumptions of the Resource Based View of the Firm, we shed light into the black box. Within a longitudinal case study, we present the motivation of a company outsourcing its recruitment activities since 2003 and the lessons learnt derived from the project during the last eight years opening the black box of outsourcing KIBP. Human Resources (HR) belongs to the back office function of organizations and recruitment in particular is considered to be a KIBP. The results of the case study indicate that business process standardization, identifying knowledge intensive sub-processes that provide value to the process outcomes and defining explicit service levels are key success factors for KIBP outsourcing. In addition, the results show that outsourcing KIBP can be beneficial for organizations and can results in better process determinants in terms of cost, time and quality.