The critical energy for trapping was defined and was then calculated for five gases (N2, Ar, CO,CO2, andN2O). The gas and the solid on which it is trapped were of the same chemical species. The calculations were made using measured values of the capture coefficient which came from cryopumping data [J. P. Dawson and J. D. Haygood, Cryogenics5, 57 (1965)]. The calculated values were obtained from an interpretation of experimental results; they were not derived from calculation of the energy and momentum exchange between a gas particle and a perfect lattice. Capture coefficients were available for a range of both surface and gas temperatures. Two cases were treated; in the first the capture coefficient was independent of surface temperature while the second case considered the surface temperature dependent values of the capture coefficients. For the first case, it was found thatEc/Dwas of the order of 1, whereEcis the critical energy for trapping andDis the depth of the surface potential well. For the second case, the relationshipEc=Aexp(B/Tsurface)fitted the data for all five gases.