Low molecular weight cyanated poly(ether sulfone) (CPES, Mn = 3200) was prepared, and cured with bisphenol A dicyanate (BPADCy). The resulting polycyanurates of different compositions show an S-shaped T-g-composition curve. This unexpected S-shaped curve is interpreted in term of the polar interaction between poly(ether sulfone) and s-triazine rings formed by polycyclotrimerization of aromatic dicyanates. Separate study on u.v. spectra of mixture model compounds suggests that this polar interaction belongs to an n-pi* interaction between the lone pair electrons of the N-atoms in the s-triazine ring and the pi*-orbital of the phenylene rings neighbouring to the sulfone linkage. I.r. study on the cured polycyanurates indicates that this polar interaction causes the shift of the -C = N stretching from 1564 to 1580 cm(-1).