Continuing the synthesis of a series of dronic acid derivatives from the corresponding carboxylic acid and P-reagents at 85 °C in methanesulfonic acid, an anomalous situation was found during the preparation of 3-phenylpropidronate. The reaction of 3-phenylpropionic acid with 3.2 equivalents of phosphorus trichloride led only to 2% of the expected product. However, using 2 equivalents phosphorous acid together with 2 or 3 equivalents of phosphorus trichloride, the target dronate could be isolated in yields of ca. 51%, suggesting the involvement of the (HO)2P-O-PCl2 ⇆ (HO)P(O)H-O-PCl2 intermediate. The best yields (62/65%) obtained using the P-reagents in a ratio of 3:2 and even 4:2 justified also the presence of intermediate (HO)2P-O-PCl-O-P(OH)2 ⇆ (HO)P(O)H-O-PCl-O-(HO)P(O)H. Similar results were obtained, when the starting material was 3-phenylpropionyl chloride, or sulfolane served as the solvent. The reactivity of the (HO)2P-O-PCl2 species towards 3-phenylpropionic acid, the corresponding chloride, and anhydride formed with methanesulfonic acid was evaluated by B3LYP/6-31G(d,p)//PCM(ACN) calculations.