We examine the effects of nanoparticle addition at low concentration on the evaporation kinetics of droplets in the constant radius mode. The evaporative behaviour of deionized water and Al2O3 nanoparticle laden water on an aluminium substrate was observed at atmospheric and at different sub-atmospheric pressures. The two fluids exhibit the same evaporative behaviour, independent of the droplet volume or the sub-atmospheric pressure. Moreover, the linear relationship between evaporation rate and droplet radius, initially proposed by Picknett and Bexon nearly four decades ago for droplets evaporating in the constant radius mode, is satisfied for both liquids. In addition we have established a unified correlation solely function of fluid properties that extends this relationship to any sub-atmospheric pressure and fluid tested. We conclude that the addition of a small quantity of nanoparticles to the base fluid does not modify the kinetics of evaporation for pinned volatile droplets.