High purity, uniform, and conformal aluminum phosphate (AlPxOy) thin films were deposited by atomic layer deposition (ALD) between 25 and 300 °C using supercycles consisting of (i) PO(OMe)3 dosing combined with O2 plasma exposure and (ii) AlMe3 dosing followed by O2 plasma exposure. In situ spectroscopic ellipsometry and mass spectrometry were applied to demonstrate the ALD self-limiting behavior and to gain insight into the surface reactions during the precursor and coreactant exposures, respectively. Compared to earlier reported AlPxOy ALD studies using H2O and O3 as coreactants or without using coreactans, the use of an oxygen plasma generally leads to higher growth per cycle values and promotes phosphorus incorporation in the film. Specifically, when using a 1:1 POx:Al2O3 cycle ratio and a substrate temperature of 150 °C, the growth per supercycle is found to be 1.8 Å. The [P]:[Al] atomic ratio for this process is approximately 0.5 (∼AlP0.5O2.9) and can be tailored by changing the ratio between the two cycles or the substrate temperature. In literature reports where the same aluminum precursor was used, the [P]:[Al] atomic ratio was limited to 0.2 or a very high number of phosphorus cycles was needed in order to increase the phosphorus content. Instead, we demonstrate deposition of films with a composition close to AlPO4 by using a 2:1 POx:Al2O3 cycle ratio. The limited incorporation of P in the film is suspected to derive from the steric hindrance of the relatively bulky phosphorus precursor. Mass spectrometry suggests that the PO(OMe)3 precursor chemisorbs on the surface without the release of reaction products into the gas phase, whereas Al(Me)3 already undergoes methyl ligand abstraction upon chemisorption.