As reported previously, while a hydrodenitrogenation-selective FeMo sulfide catalyst maintained its activity in a 600 h accelerated aging test, the catalyst lost its hydrodenitrogenation activity fairly rapidly during the early stages of the test. This study is aimed at identifying the origin of the initial activity loss. Characterization of fresh and lined-out catalysts has indicated that the initial activity decline appears to be caused by decomposition of a single-phase amorphous FeMo sulfide.