The multienzyme glycine cleavage system (GCS) converts glycine and tetrahydrofolate to the one-carbon compound 5,10-methylenetetrahydrofolate, which is of vital importance for most if not all organisms. Photorespiring plant mitochondria contain very high levels of GCS proteins organized as a fragile glycine decarboxylase complex (GDC). The aim of this study is to provide mass spectrometry-based stoichiometric data for the plant leaf GDC and examine whether complex formation could be a general property of the GCS in photosynthesizing organisms. To this end, we investigated the mitochondrial content, the stoichiometry and the isoprotein composition of the Arabidopsis thaliana leaf GDC in comparison with that of Pisum sativum using stable-isotope-labeled peptides (QconCAT and synthetic labeled peptides) and compared the results to label-free, Hi3 peptide quantification.