Given the legal status of MDMA (3,4-methylenedioxymethamphetamine), or Ecstasy, face-to-face access to participants is sometimes difficult. The number of participants in studies of cognitive performance amongst Ecstasy users is variable, with the average being around 30. Access to a larger number of participants is clearly desirable. The present investigation accessed a larger sample size using a web-based design. A website was developed and used for data collection. Prospective memory ability was assessed using the Prospective Memory Questionnaire. Self-report of day-to-day memory performance was investigated using the Everyday Memory Questionnaire. The Drug Questionnaire assessed the use of other substances as well as Ecstasy, allowing a regression design to isolate the contribution of each substance to any variance on the cognitive measures. Preliminary findings (N = 488) indicate that there is a clear double dissociation between the impact of Ecstasy and cannabis. We found that cannabis was associated with reports of 'here-and-now' cognitive problems in short-term and internally cued prospective memory. In contrast, Ecstasy was associated with reports of long-term memory problems, which were more related to storage and retrieval difficulties.