An automatic recording microbalance with a sensitivity of 1 μg and operated in a vacuum of 10^-9 torr was used to investigate the formation of Cs-Sb layers. Homogeneous layers with sensitivities of 20-30 μA lm^-1 were prepared at 140-160°C from antimony films with surface densities of 9-15 μg cm^-2. The maximum composition reached was Cs₃Sb but this was unstable and the composition slowly reduced to Cs₂Sb when baking was continued without further supply of caesium. The photoemissive yield depended on the bulk composition of the layer and also on the presence of caesium adsorbed on the surface. Optical transmission measurements indicated that the optical absorption of the layer was greatest at Cs₃Sb. Resistance maxima were obtained at Cs: Sb ratios of 3: 7, 1, 2 and 3.Investigation of the effect of oxygen on the completed surface showed that less than a few monolayers was sufficient to sensitize the layer to maximum yield.