Post-acquisition integration matters for overall M&A outcome. However within this phase researchers have struggled to identify clear links between integration activities and post-acquisition outcome. This may be due to using organisational levels of analysis, where sub-organisational issues serve to confound findings. In order to unpack the post-acquisition phase, and to delve more deeply into organisations, this paper adopts a more granular perspective on integration activities by focusing upon the building blocks of organisations. Specifically we investigate ordinary routine amalgamation and their impact upon meta-routine outcome during acquisition integration. Drawing upon two longitudinal integration cases and using ‘retroductive’ analysis, two types of amalgamation are identified, namely ‘combination’ and ‘superimposition’. We find that, while the basic nature of routines, such as multiplicity and nestedness, inhibit routine amalgamation, external interference in the form of context, structural change or introduction of additional routines is needed to stabilise amalgamated routines. From our findings we are able to suggest a number of testable propositions about the factors that influence the amalgamation of routines. This empirical study contributes to the M&A literature by opening up the ‘black box’ of post-acquisition integration by providing details at a granular level of what actually happens during integrations.