In a field experiment using the selective sorting of household wastes as the dependant variable, we have tested the combined effect of two techniques that induced compliance to a request: the foot-in-the-door (FITD) technique and the "but you are free ..." technique. Homeowners were asked to record on a form their entire household wastes for 1 month. In the FITD technique, participants were first asked to respond to a short survey on selective sorting habits. In the "but you are free ... " condition, the request for participation included a sentence that stated that the participant was free to accept to participate or not: "Of course you are free to accept or to refuse." In the combined FITD/"you are free ..." condition, the first request was addressed to the participant and the sentence that stated that the participant was free appeared in the second request. In a control condition, the final request was addressed without any mention that the participant was free or not to accept. Results found that the combined FITD/"you are free ..." condition was associated with greater compliance in completing the survey (78.%) than the single FITD (60.0%) and the "but you are free ..." condition (56.0%), whereas each of these three experimental conditions elicited greater compliance to complete the survey than in the control condition (40.0%). Commitment theory and practical interest of this combined technique for ecology are discussed.