Atomic layer deposition (ALD) is a deposition technique that is considered as primary candidate to fulfill the stringent requirements for ultrathin film growth in applications such as solar cells, displays, and semiconductors. Developing and optimizing ALD processes and improving material properties require a fundamental understanding of the reaction mechanisms, which can be obtained through studying ALD processes in situ. Transmission infrared spectroscopy can be used to study multiple aspects of atomic layer deposition processes. In this thesis, the merits of this technique have been demonstrated by investigating the surface reactions of Al2O3 and Er2O3, i.e., detection of surface groups and volatile reaction by-products during ALD, and by investigating the film composition. Thermal ALD of Al2O3, which has application potential for film growth at temperature sensitive substrates, is hampered at deposition temperatures below 150 °C due to the reduced thermal energy. To investigate the reaction mechanisms for deposition temperatures between 25 - 150 °C, the formation of the reaction by-product CH4 during both ALD half reactions has been measured by means of transmission infrared spectroscopy. It has been demonstrated that quantitative information on the formation of reaction by-products during ALD can be deduced by calibrating the absorbance from volatile CH4. A decreasing formation of CH4 per cycle revealed a lower growth rate with decreasing deposition temperatures. A direct comparison with plasma-assisted ALD of Al2O3 showed that more CH4 was produced during precursor adsorption of thermal ALD and saturation of the surface reactions progresses slower. In this respect, residual water is expected to play an important role, especially during thermal ALD at these low deposition temperatures. Furthermore, the amount of CH4 created during precursor adsorption relative to the amount in a full cycle indicates the precursor molecules adsorb bi-functionally at deposition temperatures below 150 °C. Plasma-assisted ALD of Er2O3 has been characterized and the reaction mechanism has been studied to develop Er-doping of Al2O3 by means of plasma-assisted ALD. To this end, several diagnostics have been used in situ: transmission infrared spectroscopy to detect surface groups and reaction by-products, optical emission spectroscopy to detect reaction by-products during plasma exposure, and spectroscopic ellipsometry to determine the film thickness. Differential infrared spectra of the surface groups show precursor adsorption, and removal of the precursor ligands upon plasma exposure. No reaction by-products were detected during precursor adsorption and it is therefore suggested that the Er(thd)3 adsorbs non-dissociatively. Furthermore, it is concluded that the O2 plasma reacts with the surface groups in a combustion-like reaction. In addition, Er-doping of Al2O3 has been achieved by alternation of Al2O3 and Er2O3 ALD cycles. The doping process has been actively monitored by measuring the film thickness in situ using spectroscopic ellipsometry.