Three stable oil-in-water emulsions of different drop sizes were prepared. Radical-probe reactivities with Trolox and pyrene were evaluated in the emulsions. A cut-off effect was observed for the pyrene quenching by the amphiphobic probes. No cut-off effect was observed for the hydrogen-abstracting reaction with Trolox. A cut-off effect requires partitioning of reagents between distinct pseudophases.