The effectiveness of the single session web-based brief alcohol intervention ‘What Do You Drink’ in reducing alcohol outcomes was evaluated among: 1) 907 heavy drinking college students (456 experimental group: WDYD; 451 control group: no intervention) aged 18-24 year using an Ecological Momentary Assessment (EMA) approach with multiple weekly follow-up time-points (30) and 2) 607 heavy drinking adolescents and young adults (318 experimental group; 291 control group) aged 15-20 years with a low educational background at one and six months follow-up. There were no main intervention effects in weekly alcohol consumption (WAC), frequency of binge drinking (BD), and heavy drinking (HD) among students at one and six months follow-up. However, WDYD had a preventive effect on WAC on the short term for students who 1) are ready to change drinking behaviour, 2) show more severe symptoms of alcohol abuse or dependence, or 3) participate in carnival. Moreover, WDYD was effective in preventing an increase in WAC and BD among students directly after the intervention using EMA and latent growth curve modelling techniques. These effects sustained at three and six months follow-up, respectively. WDYD was not effective in reducing WAC, BD, and HD among heavy drinking adolescents and young adults with a low educational background at one and six months follow-up.