Quadrupole mass spectrometry was carried out to detect and identify the reaction products during atomic layer deposition (ALD) of lithium carbonate (Li2CO3). We examined gas phase species for thermal ALD using a LiOtBu precursor together with H2O and CO2 and plasma-assisted ALD using the same lithium precursor combined with an O2 plasma. For both processes it was concluded that in the first half-cycle the LiOtBu chemisorbs on the surface by an association reaction of the complete precursor whereas in the second half-cycle the organic ligand is abstracted as tert-butanol. The differences between the two processes lie mainly in the formation of CO2 and H2O reaction byproducts in the second half-cycle when an O2 plasma is used as coreactant instead of H2O. The generation of CO2 supports the fact that itis possible to deposit Li2CO3 films directly by plasma-assisted ALD. Instead, in the case of thermal ALD, an additional CO2 dose step is required to deposit Li2CO3 and suppress LiOH or Li2O formation. The reaction with CO2 appears to be reversible at higher deposition temperatures (T≥250°C) and by using extended plasma exposure times, and therefore the composition of the plasma-assisted ALD films can be varied between Li2CO3 and Li2O.