Some years ago the physical presentation of Paraloid B 72 changed from white, irregular lumps into regular, transparent globules. Refractive index, viscosity grade and solubility properties indicate that the chemical composition of the new and the old B 72 could be different. This was confirmed by pyrolysis-gas chromatography and mass spectrometry. Both resins are copolymers of methyl acrylate and ethyl methacrylate, but the old B 72 contains 2% more methyl acrylate than the new. Since this small change in composition results in some distinct changes in physical properties of the resins it is advised to repeat some of the aging tests carried out before on the old B 72.