A quantitative model is derived for the progress of a nucleation and growth controlled reaction during heating at a constant rate. It incorporates nucleation, growth, and impingement, takes account of temperature dependent solubility, and distinguishes between diffusion controlled growth and linear growth. Parameters of the model for a single process reaction are : s, which is akin to the Avrami parameter for isothermal analysis, E-eff the effective activation energy, eta(i) the impingement parameter, and a pre-exponential constant. The model is applied to the DSC data on precipitation in Al-Si alloys. An excellent agreement between data and model is obtained. The parameter s is interpreted in terms of the processes taking place, namely growth of small nuclei, growth of coarse particles or nucleation and growth.