An approximate method of estimating the maximum saturation, the nucleation rate, and the total number nucleated per second during the laminar how of a hot vapour-gas mixture along a tube with cold walls is described. The basis of the approach is that the temperature and vapour concentration differences between the wall and a point in the tube are approximately proportional to R(r)Z(z) and R(r)F(Z(z)), respectively, where r and z are dimensionless radial and axial coordinates and R, Z, and F are known functions. Key parameters determining the location and magnitude of the maximum saturation are the Lewis number, Le, and a parameter, B, which determines the temperature variation of the equilibrium vapour concentration (B is roughly the latent heat of vaporisation per vapour molecule divided by Boltzmann＇s constant). For vapour-gas mixtures with Le > 1, the maximum saturation occurs at the tube axis but for those with Le < 1 it occurs near the tube wall. The magnitude of the maximum saturation achieved increases rapidly with B. The approach assumes that condensation on previously nucleated particles is negligible, and a condition on the maximum nucleation rate for this to be so is derived. Predictions are compared with numerical calculations for DBP vapour and for water vapour and very good agreement is found. The approach is used in conjunction with Hale＇s scaled nucleation theory to determine the tube wall-inlet temperature difference needed to achieve various nucleation rates for any specified vapour-gas mixture. This approach can be applied to many other two-dimensional systems where simultaneous heat and mass transfer occur provided that the temperature and vapour concentration can be expressed in the forms given above. Crown