In the present investigation, 99 compulsory school students in Sweden were asked to define nine English idioms chosen for their variation in inherent transparency and frequency. The participants were divided into two groups, where Group 2 was provided with context of varying degrees of support, while Group 1 was not. All students answered a biographical questionnaire. Thus, the effects of intrinsic (transparency and frequency) and extrinsic factors (contextual support, extra-curricular activities (ECAs) and age) could be examined. Results show that transparency and frequency do affect idiom comprehension, but to a much lesser extent than context. A tentative correlation between more time spent on ECAs and high scores could be found as well as between active/passive activities and high/low scores. Age did not seem to affect the participants’ idiom comprehension.