Intentional binding describes the phenomenon that actions and their effects are perceived to be temporally approximated. We introduced a new method of duration estimation to the research field, the method of constant stimuli. Participants freely chose to press one of two keys or experienced passive key presses. After an interval of 250 ms or 600 ms a visual effect occurred. In Experiment 1, each key produced an effect after a specific interval. In Experiment 2, both keys produced an effect after the same interval that varied between sessions. Participants compared the duration of the action-effect interval with a tone of varying duration. To assess intentional binding, we compared the perceived duration of the action-effect interval between the active and passive condition. We showed intentional binding for 600 ms, but not for 250 ms action-effect intervals in both experiments. Thus, the method of constant stimuli is suitable to assess intentional binding.