Complexes of doxycycline (I) with HPO3 and NaPO3 or KPO3, prepd. by adding I base to predetd. excess of HPO3 in CHCl3-MeOH, then neutralizing the excess HPO3 with NaOH or KOH, had improved soly. [comparable to doxycline hyclate (II)], gave better retention and higher levels in the blood stream than II or other known I salts, had a better taste than II, and the metaphosphate groups blocked inactivation of I by chelation with Ca2+ or Mg2+. Complexes thus prepd., solns. of which had high stability, were: I.1/5NaPO3.HPO3; I.1/2NaPO3.HPO3; and I.1/5KPO3.HPO3.