This study investigated students' allocation of responding as a function of task difficulty and type of reinforcement contingency (i.e., accuracy based or time based). Four regular education fourth-grade students were presented with two identical stacks of easy and then difficult math worksheets using a reversal design. Regardless of condition, completing problems from each stack of worksheets was reinforced according to a different contingency; one required correct completion of math problems (accuracy based) and one required on-task behavior (time based). Results suggested that 3 of the 4 students preferred the accuracy-based contingency when given easy material and the time-based contingency when given difficult material. One student allocated more responding to the accuracy-based contingency when given easy problems but did not show a clear preference for either contingency with difficult problems. The implications of these findings for designing reinforcement-based programs for tasks of varying difficulty are discussed.