We describe a study of programs to deepen families' scientific inquiry practices in a science museum setting. The programs incorporated research-based learning principles from formal and informal educational environments. In a randomized experimental design, two versions of the programs, called "inquiry games," were compared to two control conditions. Inquiry behaviors were videotaped and compared at pretest and posttest exhibits. Family members were also interviewed about their perceptions and use of the inquiry games. Results indicated that visitors who learned the inquiry games improved their inquiry more than those who did not. Effect sizes ranged from 0.3[sigma] to 0.7[sigma], depending on the assessment measure. Visitors who learned the collaborative inquiry game showed even more improvement than those who learned the individualized game, spending more time investigating the posttest exhibit, making more frequent and more abstract interpretations of their experiments, building more collaborative explanations, and engaging in more coherent inquiry investigations than controls. Qualitative analysis suggested that the collaborative inquiry game was superior because it required all family members to participate, work together, and explicitly articulate their interpretations. Visitors in all conditions enjoyed their experience, varied in what they liked and disliked, and reported applying what they had learned at new exhibits. (Contains 8 figures, 5 tables, and 7 footnotes.)