Explicit instruction is a pedagogical approach in which students are taught step-by-step procedures for solving problems and practice using the procedures. Despite considerable evidence supporting the use of this general approach, relatively little is known about how varying specific design features impact its effectiveness. This preregistered study investigated the impact of two design features, explaining rationales for procedures and practicing in interleaved sequence, on the efficacy of explicit instruction in fraction arithmetic. Participants were 156 sixth to ninth grade children. Children received a brief intervention involving explicit instruction in fraction arithmetic procedures. They were randomly assigned to receive or not receive explanations of rationales for the procedures, and to practice different procedures in either interleaved or blocked sequence. Relative to pretest, accuracy improved on immediate and delayed posttests (Cohen’s d = 0.88 and 0.55 respectively). The magnitude of this improvement was not affected by either experimentally-manipulated factor or by both of them together. The results support the use of explicit instruction in fraction arithmetic, but provide no evidence that explaining rationales for procedures, or interleaving practice thereof, increases the effects of such instruction on procedural knowledge.