In this work, we studied ALD of HfS2 from the two precursors TDMAH and D2S on two atomically defined CoO surfaces, namely CoO(100) and CoO(111), under UHV conditions. In particular, we scrutinized the nucleation mechanism and surface chemistry in the first half cycle, i.e. upon reaction of the CoO surfaces with the Hf precursor. We monitored the processes in-situ by time-resolved IRAS. Our main findings can be summarized as follows: