Resistivity measurements and small-angle scattering of X-rays were employed to study the effects of (10.5 MeV.) deuteron irradiation on the formation of spherical coherent Guinier-Preston zones (G.P. zones) in Al-5.3 at. % Zn. Specimens were irradiated at 77°K after different thermal treatments: asquenched from 300°C, as-reverted from 200°C, and as-reaged. Irradiation of the specimen quenched from 300°C has the effect of slightly retarding the rate of zone formation. For specimens reverted at 200°C and then irradiated with 0.5 to 1.9 × 10 15 deuterons/ cm 2, the initial part of the reaction is accelerated, the rate of zone formation increasing with dose. The later part of the reaction is not greatly influenced, the final zone sizes, as determined using small-angle scattering of X-rays, are only slightly larger. Irradiation of an alloy containing zones (as formed on reaging) does not result in enhanced growth. It is concluded that vacancies as generated by irradiation with deuterons can aid G.P. zone formation, hut the defect distribution as obtained by quenching is different from that resulting from irradiation. For the case of the present alloy, deuteron bombardment does not give rise to enhanced nucleation.