The thermal behaviour of yttrium(III) isovalerate (Y(C$_4$H$_9$CO$_2$)$_3$) was studied in argon by means of thermogravimetry, differential thermal analysis, FTIR-spectroscopy, hot-stage optical microscopy and X-ray diffraction with a laboratory Cu-tube source as well as with a synchrotron radiation source. Two structural transitions take place in the solid state at 100 °C and 140 °C. They are followed by the decomposition of the isovalerate salt with release of gaseous products consisting of CO$_2$ and 2,6-dimethyl-4-heptanone and formation of Y$_2$O$_2$CO$_3$ between 320 °C and 440 °C. Above 440 °C, Y$_2$O$_2$CO$_3$ is slowly converted to Y$_2$O$_3$ with release of CO$_2$. The decomposition is complete at about 900 °C.