Theoretical considerations and experimental results show that the pumping effect of a mercury-vapour pump, usually called a `diffusion pump', is not explained by Gaede's theory of the diffusion principle. Starting from Langmuir's condensation principle the same considerations lead to a different theory; a new pump has been designed on the basis of this theory. This pump has a volumetric pumping speed of the order of 1000 1./sec., and its effective working range extends from the lowest pressures up to 10-1 mm. of mercury.