There is disclosed a method and apparatus for the production of ammonium polyphosphate solutions of high concentrations from phosphoric acid and ammonia. The process is ideally suited for the production of such solutions from wet-process phosphoric acid which has a P.sub.2 O.sub.5 content of from 56 to about 70 weight percent, impurity-free basis, and which, preferably, contains from 5 to about 50 percent of its phosphorus present as polyphosphoric acid. The method comprises introducing the phosphoric acid into a reactor bearing a Teflon liner and reacting it therein with vapors of ammonia while controlling the amount of ammonia from 0.2 to about 0.3 pound ammonia per pound of P.sub.2 O.sub.5 in the feed acid, sufficient to achieve a peak reaction temperature in the reactor between about 525.degree. and 775.degree.F., the absolute value of which is dependent on the bulk water content of the feed acid. The products from the reactor are quenched by direct contact with an aqueous solution of ammonium polyphosphate, additional ammonia is added to neutralize the phosphoric acid to a pH value between about 5.5 and about 8.5 and sufficient water is added to achieve a concentration of from 24 to about 55 weight percent P.sub.2 O.sub.5 in the aqueous solution. Preferably, the aqueous solution is continuously circulated, cooled and passed to a storage vessel which serves as a product accumulation and supply of circulating liquid. The preferred apparatus of the invention comprises a tubular reactor having inlet sparging means for the ammonia vapors and phosphoric acid to introduce these reactants into intimate contact at the inlet portion of the reactor. The reactor and the inlet spargers are formed of Teflon and it has been found that the Teflon provides a scale-resistant lining for the reactor surfaces such that the reactor can be operated for extended periods of time without requiring shutdown for cleaning and maintenance.