The thermal behaviour of Ln(C$_{3}$H$_{7}$CO$_{2})_{3}$ (Ln = Er, Tm, Yb or Lu) was studied in argon from room temperature by means of thermogravimetry and differential thermal analysis up to 1400 °C, by infrared spectroscopy, hot-stage optical microscopy and X-ray diffraction. Melting prior to decomposition was observed in all four compounds, but its course depends on the rare-earth element. Decomposition to sesquioxides proceeds via the formation of dioxymonocarbonates (Ln$_2$O$_2$CO$_3$) and release of 4-heptanone (C$_3$H$_7$COC$_3$H$_7$) as well as carbon dioxide (CO$_2$) without evidence for an intermediate oxobutanoate stage. During the decomposition of Ln$_2$O$_2$CO$_3$ into the respective sesquioxides (Ln$_2$O$_3$), an intermediate plateau extending from approximately 550 to 850 °C appears in the TG traces. The overall composition during this stage corresponds approximately to Ln$_2$O$_{2.8}$(CO$_3$)$_{0.2}$, but the state is more probably a mixture of Ln$_2$O$_2$CO$_3$ and Ln$_2$O$_3$. The stability of this intermediate state seems to decrease with the mass of the rare-earth elements. Complete conversion to Ln$_2$O$_3$ is reached at about 1100 °C. The overall thermal decomposition behaviour of the title compounds is different from previous reports for other rare-earth butanoates.