The high initial activity of Mo phosphide catalysts and the gradual increase in the activity during the reaction were studied during the thiophene hydrodesulfurization (HDS) at 573 K and atmospheric pressure. A series of unsupported Mo phosphides with various P/Mo ratios were prepared and reduced by temperature-programmed reduction (TPR) from 673 to 723 K (-1073 K) in a stream of H2. The rate of thiophene HDS once decreased and then increased after 2-2.5 h for several of the studied catalysts. The reduction of the Mo(60)P(40) catalyst at 923 K exhibited the highest HDS rate of the other catalysts during the initial stage. The initial HDS activity of the catalysts is due to the presence of metallic Mo reduced by the TPR preparation. The increase in the HDS activity during the reaction was induced by the 8231023 K-reduced Mo(60)P(40) and Mo(50)P(50) due to the formation of deficient phosphorus MoP from MoP with the phosphorus loss. The reduction and subsequent sulfidation increased neither the initial HDS activity nor the increase in the HDS activity.