Alcohol and marijuana use expectancies are presumed to be drug-specific, but prospective study of this assumption is lacking. In addition, these associations may operate differently for adults with attention-deficit/hyperactivity disorder (ADHD) histories, as expectancies have been found to be less associated with alcohol and marijuana use among this population. The first aim of the present study was to investigate whether associations between alcohol and marijuana expectancies and substance use were specific to the substances they assess. The second aim was to determine whether these associations differed as a function of ADHD history. Participants (N = 491; 281 ADHD, 210 non-ADHD) were young adults followed longitudinally between ages 21 to 23 and 29 as part of the Pittsburgh ADHD Longitudinal Study (PALS). Autoregressive models were estimated separately for positive and negative expectancies for frequency of alcohol and marijuana use and compared between ADHD groups. Although there were exceptions, results generally support the specificity of associations between outcome expectancies and respective substance use both concurrently and prospectively, but this specificity was primarily present for those without a history of ADHD. These findings suggest that young adults perceive and respond distinctly to the effects of alcohol and marijuana, but a history of ADHD may interfere with this process. These findings also extend our prior cross-sectional findings that expectancies are less associated with alcohol and marijuana use for individuals with ADHD histories. Additional research examining implicit cognitions is needed to further examine risk for substance use among those with ADHD histories. (PsycINFO Database Record (c) 2019 APA, all rights reserved).