A time-temperature-transformation (TTT) diagram has been determined for a copper-bearing steel, which is a low alloy TMCP variant of the ASTM A710 type of structural steel. Quantitative measurements have been supplemented by optical microscopy and transmission electron microscopy to investigate the isothermal transformation behaviour as well as the associated precipitate morphologies. It is shown that the kinetics and product phases of the polymorphic transformation and precipitation reactions are sensitive to both temperature and time. The TTT-diagram shows a prominent transformation region for bainitic structures, at temperatures intermediate between those of polygonal ferrite and martensite. In the intermediate region, the microstructures were characterised by a ferritic matrix with a lath and/or plate shaped grains containing a high dislocation density, together with a minor dispersed "island" phase. For a short holding time (5sec) at intermediate temperatures (580-430°C), the island phase was identified as untempered twinned and lath martensite, autotempered twinned and lath martensite, and martensite/austenite constituent, depending on the level of carbon partitioning in the remaining austenite before quenching in water. For a longer holding time, the carbon enriched austenite regions decomposed to carbide and ferrite by coupled growth. Polygonal and quasi-polygonal ferrite were formed at relatively high transformation temperatures and these microstructures contained a low dislocation density and were associated with interphase ε-copper precipitates.