Alkali polyphosphate complexes of doxycycline, (C22H24N2O8)z.(MPO3)x.(HPO3)y where M = Na or K, z = 1-5, x = 1-5, y = 1-5, x + y .ltoreq.6, and z .ltoreq. y, were prepd. and had water soly. >4.5 mg/ml. E.g., NaOH (0.5 g) dissolved in 10 ml MeOH was added to a soln. contg. metaphosphoric acid (8 g) in 200 ml 1:1 MeOH-CHCl3 and to this, doxycycline (23.1 g) in 100 ml EtOH was added. The resultant soln. was stirred for 1 hr and 350 ml iso-PrOH was added to ppt. (C22H24N2O8)3.(HPO3)5.NaPO3 [60022-06-2], m.p. 185-9.degree.