Open Access

Purpose. Mergers & acquisitions (M&As) can be an effective way to expand into new markets or business opportunities. Yet, a considerable number of failed M&As can be attributed to disregarded human resource (HR) concerns. In particular, an organization’s leadership tends to hail the advantages of a merger or acquisition during the early stages, raising employees’ expectations (honeymoon effect). Many documented failures in such corporate transactions indicate organizational members’ declining satisfaction following a deal (hangover effect). Design/methodology/approach. Drawing on in-depth interviews with senior M&A experts at a global big-four accountancy firm and focus group sessions with their respective clients, this study investigates in two cases the interplay between HR issues and M&A transactions and infers effective risk management actions. Findings. A honeymoon hangover after a transaction may appear in organizations if HR issues are neglected. Study results provide notable implications for HR departments and HR professionals facing a merger or acquisition. These implications include (1) focusing on HR risks, (2) involving HR executives to manage the HR due diligence efforts, (3) setting up transition teams that communicate well, (4) creating policies for learning and knowledge sharing, (5) developing new competencies for the NewCo, (6) being sensitive to cultural differences and (7) considering legal aspects. Originality/value. Although M&As have been much researched, relatively little has been written on practical managerial adaptation from a human resource perspective and its implications for organizational learning. This article helps address this imbalance by providing a people-oriented approach for effectively managing M&As from beginning to integration. Research limitations/ implications. The two transactions studied revealed patterns that are important for successful change. However, we should not underestimate the individual perspective in M&As. Further studies with interview data directly from stakeholders are important to analyze further the relationships between HR due diligence, organizational learning, effective knowledge transfer, and culture. Due to our research approach, we cannot claim that the results can be generalizable to all major M&As. Further research is needed to measure the impact of the HR Due Diligence aspects outlined on M&A success.