This paper attempts to investigate the influence of compression work on free convective flows in gases. Since this effect is usually small a dimensional analysis using the complete set of governing equations is presented first, in order to recognize the cases for which compression work is not negligibly small in comparison with other effects. Among other things it is shown that in gases viscous heating is always of a much lower order of magnitude than compression work. Next a case of boundary layer flow that possesses similarity is studied in some detail. For various degrees of importance of compression work a numerical integration of the governing ordinary differential equations is carried out. For very strong compression work an asymptotic solution can be found. In all cases the boundary layer shows oscillatory behaviour of both the temperature and velocity profile.