Abstract The time-dependent development of convective intrusions in a thermohaline stratification is investigated using a composite grid method with local refinement. An imposed constant sidewall heat flux causes convective cells to form at the heated wall, which subsequently propagate into the bulk domain. For the composite grid computational method, a grid interface treatment based on strict local flux conservation alone allows arbitrary, nonphysical jumps in the temperature and salinity across block boundaries for certain boundary conditions. A revised treatment based upon a linear interpolation with conservative correction is employed to overcome this difficulty. Detailed features of the internal intrusion structure are captured and the sharp interfaces between neighboring intrusions are handled with fine resolution. Two classes of thermohaline intrusions corresponding to recently-obtained experimental results have been computed. Comparison with these results shows excellent agreement both qualitatively, in terms of the internal structure of the intrusions, and quantitatively, in terms of the physical size of the intrusions and the intrusion front propagation speeds. Fundamental differences in the merging processes for the two intrusion classes which were observed in the experiments have also been observed in the numerical results.