Polydimethylsiloxane containing hydroxyl end groups can be vulcanized at room tern-perature by using ethyl orthosilicate as crosslinking agent and dibutyltin dilaurate as catalyst．The generally accepted mechanism of the vulcanization is given in Eq．(1)．However，in actual practice，the relative quantity of ethyl orthosilicate to hydroxysiloxane used is always many times larger than the stoichiometric ratio．The excess of ethyl orthosilicate should render the chain terminal unreactive toward the crosslinking reaction as shown in Eq．(2)，thus questioning the validity of the proposed mechanism．In our experiments carried out in moist air(Table 1)，the rate of vulcanization in-creased with the increase of the amount of ethyl orthosilicate，and the reverse was true when vulcanization took place in dry sealed tubes(Table 2)．This indicates that under dry condition，the excess ethyl orthosilicate retardS the crosslinking action as expected，while，in the presence of water，ethyl orthosilicate is hydrolysed to form polyethylsilicate having increased opportunity for the crosslinking．Consequently，the vulcanization should proceed according to Eqs．(3) and (4) where polyethylsilicate，instead of ethyl ortho-silicate，is the real crosslinking agent，as is generally proposed．When heated to 250℃ in air，the tensile strength of the room temperature vul-canized silicone rubber dropped to almost zero within an hour，and then gradually re-covered on prolonged heating (Fig．1b)．The recovery was not observed in N2 atmosphere (Fig．1c)．Similar results were obtained when polydimethylsiloxane alone was heated with dibutyltin dilaurate．In air，the viscosity at first dropped and then rose again (Fig．3a)，whereas，in N2，there was no such increase in viscosity after the initial drop (Fig．3b)．A small amount (0.1％) of dibutyltin dilaurate caused a significant decrease of molecular weight of polydimethylsiloxane on heating，while further increase of the catalyst produced only a small effect (Fig．4)．However，when water was added，great decrease in molecular weight was noted (Fig．5)．The effect of increasing amount of laurie acid on the molecular weight is shown in Fig．6．All these seem to indicate that the initial drop of the tensile strength of the room temperature vulcanized silicone rubber is due t0 the breaking of the Si—O—Si bond of polydimethylsiloxane by a trace of water in the system under the catalytic action of laurie acid or of dibutyhin dilaurate (Eq．5)．The recovery of the tensile strength when heated jn air for a long time iS caused bv the oxidation of the methyl group of the dimethylsiloxane in the presence of dibutyltin dilaurate to form new crosslinking units．