The sorption isotherms for three organic solvents on PVAc have been measured via the isopiestic method using a simple inexpensive device with a spring inside the measuring chamber as the force sensor. A robust steel spring was used successfully instead of the sensitive quartz spring described in the literature. The sample was a thin aluminium foil coated with a thin polymer layer to yield a short diffusion path length and sufficient equilibrium load at an acceptable tare mass. This enabled equilibrium to be attained within 1-2 h. The experimental data may be well described by the Flory-Huggins theory. The interaction parameter was fitted as a linear function of the volume fraction. Temperature was of no significant influence within the range 20-50 degrees C or at 60 degrees C, respectively. The sorption isotherms can also be predicted by the UNIFAP (UNIFAC for polymers) method without the use of experimental data. PVAc exhibits highly sorptive behaviour, e.g. 10 wt.% methanol at 80% activity. The measuring device employed may be useful for the study of adsorbents that can be coated as a film onto aluminium foil or exist as a thin film themselves (e.g. membranes).