Poly(vinyl alcohol) (PVA) can be dissolved in a non-aqueous medium in the presence of catalytic concentrations of C2H5ONO2.DMSO(EN.DMSO). Poly(vinyl formal) (PVF) of 90% acetalization was prepared by acid catalysed homogeneous formalization of PVA of molecular weight 14000 with paraformaldehyde. The formation of the formal was confined from the i.r. and H-1 n.m.r. spectra. The molecular weight of the polymer was 104 000 as determined by g.p.c. The glass transition temperature (T-g), activation energy (E), order of reaction (n) and rate constant of melting (k) were computed from d.s.c. thermograms. T-g, E, n and k were found to be 52 degrees C, 33.29 +/- 0.71 kJ mol(-1), 0.82 +/- 0.01 and 1.29 x 10(2) +/- 1.11 s(-1), respectively. Thermal stabilities were checked by t.g.a. and d.t.g. The formal decomposed in three stages. The corresponding initial decomposition temperatures were found to be 300, 465 and above 500 degrees C. A possible mechanism for the three-step thermal decomposition of PVF is proposed.