In the four studied monoalkyl phosphoric acids (n-C 12 H 25 OPO(OH) 2 , MDP; n-C 14 H 29 OPO(OH) 2 , MTP; n-C 16 H 33 OPO(OH) 2 , MHP; and n-C 18 H 37 OPO(OH) 2 MOP), only MOP can form an insoluble monolayer at the air/water interface (pH 5.6), suggesting that the longer alkyl chain (≥C 18 ) is essential for the formation of insoluble monolayers. On the contrary, all four corresponding dialkyl phosphoric acids ((n-C 12 H 25 O) 2 PO(OH), DDP; (n-C 14 H 29 O) 2 PO(OH), DTP; (n-C 16 H 33 O) 2 PO(OH), DHP; and (n-C 18 H 37 O) 2 PO(OH) DOP) can form insoluble monolayers, with only the π–A isotherm of DDP showing a phase transition plateau at 25°C. The enhancement of the subphase temperature not only increases the plateau pressure of the DDP monolayer, but also induces the emergence of a plateau for the DTP monolayer. In contrast to the weak influence of Na + and K + (1×10 −4 M in the subphases, pH∼5.6) on the π–A isotherm of DDP, Ca 2+ , Sr 2+ , and Ba 2+ (1×10 −4 M in the subphases, pH∼5.6) have an evident impact on the isotherms of DDP, and the different isotherm results indicate that DDP can recognize the three divalent cations at the air/water interface. In addition, the gaseous portion and phase transition plateaus of the isotherms of some DAPs on pure water and on subphases containing Ca 2+ , Sr 2+ , or Ba 2+ were well simulated by Volmer's equation of state and Vollhardt's equation, except for a small difference for gas phases around critical points. The relationship between the plateau and the net molecule area is also discussed.