The constitution of austenite in thin-slab cast, low-C, Mn steel microalloyed with 0.007wt%Ti, 0.04wt%Nb and 0.011wt%V was simulated in small, laboratory castings. Two carbon levels, 0.007wt% and 0.22wt%C, each at two nitrogen levels, 0.003wt% and ∼0.013wt%N, were investigated. After solidification the castings were quenched rapidly from ∼1400°C in order to investigate the eutectic carbonitrides. Samples of the castings were reheated to a range of temperatures in the austenite phase field to investigate the fine precipitates that then appeared, and the re-solution of the eutectic carbonitrides. The compositions of precipitates were determined using high resolution electron microscopy with EDX and PEELS, and compared with the results of a computer model of the solution thermodynamics of (Ti x Nb v v 1-x-v )(C y N 1-y ) in austenite. The eutectic carbonitride formed only in the higher-C steels independently of N content. However, increasing the nitrogen content of the steel increased the N:C ratio in the eutectic carbonitride. At both nitrogen levels the carbonitride was almost pure Nb(CN). After allowing for strong segregation of Nb and weak segregation of Ti to the liquid during freezing, these results agreed with the thermodynamic model, assuming the eutectic carbonitride to be in equilibrium with austenite. The presence of the eutectic carbonitride had little effect on subsequent precipitation in reheated austenite. The new precipitates were Nb-rich at low and intermediate temperatures, with some V at the lowest reheating temperatures. With increasing reheating temperature, first the V content and then the Nb content of these precipitates decreased, and their Ti content increased, as they also became N-rich, in approximate agreement with the computer model. At temperatures above approximately 1200°C the Ti tended to form Ti-rich Ti-Mn-oxides. The eutectic precipitates partially dissolved, but remained Nb-rich on reheating.