A piston expansion tube in combination with Mie light scattering is used to investigate growth of monodispersed water droplets due to condensation of surrounding pure vapor. The droplets are generated by homogeneous nucleation from the vapor phase and are observed in the size range 1.6-14x10(-7) m which lies in the transition regime around Knudsen number 1. A model bridging between molecular and continuous heat and mass transfer processes is formulated on the basis of the well-known three-layer concept (droplet-collision-free zone-continuum). It includes a uniform pressure hypothesis which saves using the complicated energy equation in the collision-free zone. Consistent agreement with experimental results is found when only a minor adjustment factor is used.