The phenomenon of 'gas migration' in oil well cementing is believed to occur during the transition state between initial and final set of the cement. As part of an initial study to elucidate the mechanism of gas intrusion into the cementing material, the external and total chemical shrinkage of API class G cement slurries was measured for the first 48 h of cun'ng. In order to counter the disturbance of segregation and formation of bleed water during the measurement of external chemical shrinkage, precipitated calcium carbonate (:5 15 % ,fineness 18 m 2 / g) and the viscosifier polyvinylalcohol were added. Neither addition significantly influenced the hydration rate of G cement with water-cement ratio (w/c) = 0·50 during the first 48 h. The chemical shrinkage, both total and external, of G cement slurries on hardening seems to be largely independent of w/c in the first 48 h. The external and total shrinkages of a neat G cement slurry curing at 20°C and atmospheric pressure are about 1·0 mll 100 g cement or 1·2 vol. % and 2·2 mlll00 g cement or 2·6 vol. % respectively after 48 h.