The microalloying element Nb plays an important role in the production of high-strength low-alloy steels, as it significantly influences grain refinement during the thermomechanical manufacturing process by precipitating fine carbides or carbonitrides. In this work, the solubility behaviour of Nb is therefore investigated as a function of the austenitisation temperature of two microalloyed Nb-Ti steels using atom probe tomography. In addition, various microstructural features are analysed with different etching methods using light and scanning electron microscopy. The results of the metallographic characterization of the Nital etched samples show that the initial microstructure is ferritic-pearlitic, with no differences in phase fraction and grain size between the alloys, and that after the heat treatment the microstructure is predominantly bainitic with a small amount of ferrite. Samples etched with a picric acid based etchant confirm the expected increase in austenite grain size with increasing austenitisation temperature. Atom probe measurements show that more Nb can be dissolved with increasing austenitisation temperature, with solubility increasing significantly from 950°C and approaching a plateau at high austenitisation temperatures of 1150°C and above. At austenitising temperatures of 950 and 1050°C no effect of the Ti/N ratio on the Nb solubility can be detected. Comparison of the measured results with a theoretical solubility product from literature shows increasing deviations at austenitisation temperatures above 950°C, which can be explained by the fact that this model does not take into account the presence of complex (Nb,Ti)(C,N)-precipitates or the size or distribution of the precipitates.