Synthesis of new quinoline‐(amino)methylphosphonic acids, their phosphonate esters, and phosphine oxides is presented. The desired new compounds were efficiently obtained by nucleophilic addition of phosphorous species to quinoline‐derived Schiff bases. In addition, it was discovered that heating of quinolin‐2 and quinolin‐4‐yl‐(amino)‐methylphosphonates with aqueous HCl leads to their decomposition resulting in a rupture of the CP bond, rejecting of the phosphorus containing fragment, and formation of the corresponding secondary quinoline‐2 and quinoline‐4‐alkylamines. Two alternative mechanistic pathways for this cleavage are postulated. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:617–624, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20704