Communities Mobilizing for Change on Alcohol (CMCA) and Tobacco Policy Options for Prevention (TPOP) are randomized multicommunity trials designed to reduce the availability of alcohol (CMCA) and tobacco (TPOP) products to underage youth. We assessed the test-retest reliability and internal consistency of perceived availability measures developed for surveys of middle and high school students. Two questionnaires were administered twice, each to separate student populations (CMCA: 9th graders; TPOP; 8th graders) 21 to 26 days apart. A total of 111 (CMCA) and 70 (TPOP) subjects participated at both times (participation rate of 93.3% and 84.3%, respectively). There were no statistically significant differences in the distribution of demographic variables, alcohol or tobacco use variables, or availability variables across administrations. The scales assessing perceived access to alcohol or tobacco had high internal consistency (Cronbach's alpha of 0.84 for alcohol and 0.77 for tobacco), but moderate to low test-retest correlations (0.58 and 0.13, respectively). Single-item measures of sources of alcohol and tobacco and reported buy attempts also had moderate to low test-retest correlates (0.10-0.65). We recommend that items assessing tobacco and alcohol availability to underage youth be further developed.