A general hypothesis of atom size effects for G.P. zone formation is discussed in this paper and results are presented of precipitation in the systems Fe-Au and Fe-Mo. Techniques used are resistivity measurements and electron microscopy. In the Fe-Mo system it is shown that after initial cluster formation during the early stages of ageing after the quench, further growth ceases and vacancies anneal out into dislocation loops. The activation energy for the initial clustering was 1·3 ev whilst the excess vacancy (...) concentration estimated from the loop area gave a vacancy formation energy of ∼1·5 ev. These results confirm recent quenching results on pure iron. The refusal of Mo zones to grow confirms the general hypothesis of size effects.In the Fe-Au system precipitation of gold platelets after quenching is shown to be associated with the high density of dislocations formed during quenching through the phase transformation of the iron matrix. The effect of the dislocations is shown by the low value of ∼0·5 for the Johnson—Mehl kinetic parameter and by the activation energy for growth which is ∼0·8 of the equilibrium diffusion energy for Au in Fe. As the size-effect hypothesis does not apply to precipitation on dislocations the growth of zones in Fe—Au does not affect its validity. (shrink)