Poly(phenylsilsesquioxanes) of regular ladder like structure were synthesized by condensation of [PhSi(O)OH](4) obtained in situ from sodium cyclosiloxanolate {(Na+)4[PhSi(O)O-](4)} nL (L = iPrOH, H2O). The composition of oligomers present in the reaction mixture was analysed with MALDI ToF during the course of polycondensation. Redistribution of siloxane bonds and cyclization of macromolecules were detected. Cyclic LPSQ-Ph species and silsesquioxane redistribution in basic medium as also observed for oligo(phenylsilsesquioxanes) obtained by stepwise coupling polymerization. TGA and FTIR analysis of thermal decomposition process indicated significant differences in the course of structural rearrangements at high temperatures for both types of polysilsesquioxanes. (C) 2016 Elsevier Ltd. All rights reserved.