An evaporation apparatus was developed for In Vitro evaluation of repellent evaporation from long-lasting mosquito repellent formulations. EVAP was used to evaluate mixtures of Deet with vanillin and vanillin derivatives. Those formulations, which exhibited a lower evaporation rate of Deet in EVAP studies, were found to afford a longer duration of protection against Aedes aegypti (L.) mosquitoes.