Logarithm of the diffusion coefficient as a function of the inverse of the temperature for HNCO (squares), H₂CO (circles), and NH₃ (triangles) and for CO (crosses, in the inset), as obtained from the diffusion equation. The errors bars are evaluated from dispersion measurements on NH₃ at 117 K, and are probably overestimated for CO. The temperature dependence of the diffusion coefficients is fitted against an Arrhenius equation (solid lines).