An apparatus for performing dissolved gas analysis on electrical insulating oil. The apparatus is connectable to a first oil source and a second oil source and has a respective sample cell for each source, an analysis cell and a gas analyser. Each sample cell is selectably connectable to said analysis cell to allow fluid communication therebetween, while the other sample cell is isolated from the analysis chamber. The apparatus allows dissolved gas analysis to be performed on samples from more than one source while minimizing the effects of cross-contamination between samples.