Lattice imaging and conventional transmission electron microscopy have been applied in an investigation of the relationship between grain boundary structure and boundary-nucleated phase transformations. The study encompassed three reactions associated with grain boundaries in the Al-Zn system: heterogeneous nucleation at grain boundaries, discontinuous precipitation involving the R-phase and the low temperature discontinuous reaction. In conjunction with image computations, lattice images were interpreted using direct fringe spacing measurements, microdensitometer traces and optical diffraction, providing superior capabilities in structural and chemical analysis. Errors of less than or equal to 1% were achieved by only 10-fringe samplings using an internal magnification standard.