1,4-Dioxane-2-one (DX) was polymerized in 1,4-dioxane as solvent or in bulk by means of the tin octoate/butyl alcohol (Sn(Oct)(2)/BuOH) mixture as an initiating system, in the range of temperatures from 80 to 120 degreesC. Size exclusion chromatography (SEC) and MALDI TOF mass spectrometry measurements of the crude reacting mixtures revealed that in the thermodynamically equilibrated systems a relatively high fraction of cyclic oligomers (DX(i)) appear. The molar concentration of cyclic oligomers (Sigma-i [DX(i)](eq)) increased with increasing temperature and monomer concentration in the feed and reached a maximum value equal approximately to 1 mol L-1 for the polymerization in bulk. On the other hand, the mass fraction (f) of cyclic oligomers after passing a maximum (f = 40% at [DX](0) approximate to 4.0 mol L-1, 100 degreesC) decreased with increasing [DX](0), and eventually for bulk polymerization f < 10% (f = 8% at 80 &DEG;C) was reached. The equilibrium monomer concentration, determined by means of SEC, increased with increasing [DX](0) until [DX](0) &AP; 4.0 mol L-1 when it became constant value ([DXI,,). The temperature dependence of thus-determined [DX](eq) gave the following thermodynamic parameters of polymerization: &UDelta;H-p = -13.8 kJ mol(-1) and &UDelta;S-p(o) = -45.0 J mol(-1) K-1. Thermodynamic parameters of polymerization of cyclic oligomers (DX(i), i = 3-8) have also been determined in a similar way.