In 1962, Floreen and Decker showed that the increase in hardness {Delta}H, in the early stages of hardening of an 18%Ni marageing steel, could be related to the ageing time t by an expression of the form: {Delta}H = (Kt){sup n} where K = temperature dependent rate constant, n = time exponent, slightly temperature dependent. At a later date, a similar relationship was reported by Squires and Wilson for the early stages of ageing of an Fe-12Ni-6Mn marageing type alloy. Squires and Wilson also gave a theoretical explanation of this relationship in terms of ageing kinetics and theories of coherency hardening; a brief summary is given here for the sake of completeness. The main purpose of this note is to report, and explain a relationship of the form: {l_brace}1/{Delta}H{r_brace}{sup 3} = constant {times} (t {minus} t{sub 0}) + {l_brace}1/{Delta}H{sub 0}{r_brace}{sup 3} where: {Delta}H{sub 0} = increase in hardness at commencement of coarsening time t{sub 0}. = peak hardness, H{sub p} -- as quenched hardness, H{sub 0}.