The reaction mechanism in the atomic layer deposition of Nb2O5 from Nb(OEt)(5) and deuterated water was studied in situ with a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM). The responses of these in situ measurement techniques to the characteristics of the ALD processes were thoroughly clarified and the process parameters carefully optimized. Also the effect of the reaction temperature oil the extent of decomposition reactions of Nb(OEt)5 interfering with the ALD process was investigated. Decomposition did occur at 400 degrees C but not at temperatures of 350 degrees C and below. Also the reaction mechanism was studied as a function of reaction temperature and found to be about the same until decomposition of the precursor started. Deuterated ethanol was found to be the most important gaseous byproduct of the ALD process but also some diethyl ether apparently formed. About one of the five ethoxide ligands of Nb(OEt)(5) was released during the N (OD)5 pulse and the rest during the D2O pulse. Finally separate experiments were performed to study the adsorption of ethanol as well as 2-propanol oil the surface of Nb2O5 Ethanol was found to adsorb. It could also be stated that water can replace the adsorbed ethanol. Oil the other hand 2-propanol apparently did not adsorb on the surface of Nb2O5.