Aims and objectives: The aim of this in vitro study is to assess the diffusibility and degradation of three calcium hydroxide pastes. Materials and methods: Three pastes were prepared by mixing calcium hydroxide powder with three different vehicles namely water, propylene glycol and CMCP for the investigation. The three pastes were sealed in porcelain caps and immersed in distilled water for the diffusion study. The change in the pH of the surrounding water indicated the rate and degree of diffusion in the degradation study (1 to 7 days, 14th day, 21st day and 30th day). The pastes were exposed to an atmosphere of carbon dioxide in the degradation study to assess the degree to which they can undergo degradation by carbonation. Mordant Black or Solochrome Black indicator was used for the estimation. Results and conclusion: Calcium hydroxide readily diffuses from all the three pastes. Calcium hydroxide diffuses marginally less from a paste made with propylene glycol as compared to CMCP paste. The comparative stability of Ca(OH)2 was studied and the results showed that consistently significant protection is imparted to Ca(OH)2 by CMCP over widely ranging periods.