The evaporation of sessile drops of water and n-decane placed on various substrates has been studied using a projection method. Drop dimensions and contact angle have been measured as a function of time. The first stage of the experiment corresponds to a saturated atmosphere; then, when evaporation is allowed to occur, two or three different stages appear (depending on the surface roughness). For the first of these, a model is proposed which allows us to calculate the coefficient of diffusion of the liquid vapor in air.