Polymethylsilazanes (PMS) were synthesized and used to cure silicone resins at ambient temperature. The curing degree of the silicone resins depends on the structure of PMS, and its concentration in the resin. Silicone resins of different structure and different silanol content can be cured to a satisfactory degree, if the concentration of the PMS is high enough. Silicone resins cured with PMS showed improved thermal stability in nitrogen, compared with that of the heat-cured counterparts. The mechanism of thermal decomposition of the silicone resins is discussed. The improvement of heat resistance using PMS as the curing agent is accounted for by diminished thermal rearrangement and degradation of the polysiloxane network initiated by silanol groups in the resins. (C) 2003 Wiley Periodicals, Inc.