["Environmental Policy and Governance, EarlyView. ", "\nABSTRACT\nEnvironmental policy integration (EPI) is essential for reconciling economic development with environmental sustainability, yet its temporal evolution and underlying mechanisms remain underexplored, particularly at the regional governance level. The development of eco‐industrial parks (EIPs) exemplifies this challenge as a cross‐cutting policy issue that necessitates horizontal integration of economic and environmental domains. To address these gaps, this study adopts and extends a process‐oriented analytical framework to examine EIP policy integration in the Province of South Holland, the Netherlands. Our findings demonstrate that the interplay between policy framing and subsystem involvement shapes integration outputs, namely policy objectives and instruments. We further identify hierarchical authority and reflective learning as two parallel mechanisms. Hierarchical authority drives EPI advances during periods of high political attention, and reflective learning builds administrative capacity that sustains integration beyond electoral cycles. This study provides implications that enhancing mainstreaming capacity and institutionalizing learning mechanisms are effective approaches to improve EPI.\n"]