We conducted a theory-driven process evaluation of a cluster randomized controlled trial comparing two types of complementary feeding (meat versus fortified cereal) on infant growth in Guatemala, Pakistan, Zambia and the Democratic Republic of Congo. We examined process evaluation indicators for the entire study cohort (N = 1236) using chi-square tests to examine differences between treatment groups. We administered exit interviews to 219 caregivers and 45 intervention staff to explore why caregivers may or may not have performed suggested infant feeding behaviors. Multivariate regression analysis was used to determine the relationship between caregiver scores and infant linear growth velocity. As message recall increased, irrespective of treatment group, linear growth velocity increased when controlling for other factors (P < 0.05), emphasizing the importance of study messages. Our detailed process evaluation revealed few differences between treatment groups, giving us confidence that the main trial's lack of effect to reverse the progression of stunting cannot be explained by differences between groups or inconsistencies in protocol implementation. These findings add to an emerging body of literature suggesting limited impact on stunting of interventions initiated during the period of complementary feeding in impoverished environments. The early onset and steady progression support the provision of earlier and comprehensive interventions. © The Author 2014.