First examples of phosphate ammine complexes of ruthenium nitrosyl trans-[Ru(NO)(NH 3 ) 4 (H x PO 4 )](NO 3 ) x ·(2−x)H 2 O (x=0, 1, 2) were obtained with good yields. Results of elemental analysis, X-ray, infrared, UV–Vis diffuse reflectance, and 31 P NMR spectroscopies fit well with the formulas [Ru(NO)(NH 3 ) 4 (PO 4 )]·2H 2 O (1), [Ru(NO)(NH 3 ) 4 (HPO 4 )]NO 3 ·H 2 O (2), and [Ru(NO)(NH 3 ) 4 (H 2 PO 4 )](NO 3 ) 2 (3). In all the complexes, the phosphate group is in a trans-position to NO and is monodentate coordinated to ruthenium by one of the oxygen atoms. The pK a values 2.6 and 7.2 were calculated from potentiometric titration and 31 P NMR for the reactions: trans-[Ru(NO)(NH 3 ) 4 (H 2 PO 4 )] 2+ +H 2 O⇆trans-[Ru(NO)(NH 3 ) 4 (HPO 4 )] + +H 3 O + and trans-[Ru(NO)(NH 3 ) 4 (HPO 4 )] + +H 2 O⇆trans-[Ru(NO)(NH 3 ) 4 (PO 4 )]+H 3 O + , respectively. The rate constant for the reaction trans-[Ru(NO)(NH 3 ) 4 (H 2 O)] 3+ +H 2 PO 4 − ⇆trans-[Ru(NO)(NH 3 ) 4 (H 2 PO 4 )] 2+ +H 2 O was estimated as k obs =7×10 −8 s −1 at 70°C in a dilute solution of H 3 PO 4 (∼5M).