Though process evaluation of health programs has received growing attention, few interventions have reported process evaluation over multiple years. This article describes 2 years of process evaluation (2003-04) for the Pool Cool Diffusion Trial. Pool Cool is a skin cancer prevention program designed to increase sun protection habits among children and improve organizational and environmental supports for sun protection at swimming pools. Each year, 80 telephone interviews and 40 site visits at pools across the United States were completed, to examine how fully the program was implemented and the extent of use of program components between the two study conditions. Major components of the Pool Cool program, including sun safety lessons, sun safety signs and sunscreen use, had high implementation. Between the 2 years, most of the core elements were either maintained or increased in use. There were no significant differences between the basic and enhanced conditions on implementation. Reasons given for successful implementation were the provision of a toolkit, ease of implementing the program, pool staff and children enjoying the program and the field coordinators' support. These data provide information on programmatic factors that contribute to successful program diffusion.