In the four studied nionoalkyl phosphoric acids (n-C12H25OPO (OH)(2), MDP; n-C14H29OPO(OH)(2), MTP; n-C16H33OPO(OH)(2), MHP; and n-C18H37OPO(OH)(2) MOP), only MOP can form an insoluble monolayer at the air/water interface (pH 5.6), suggesting that the longer alkyl chain (greater than or equal toC(18)) is essential for the formation of insoluble monolayers. On the contrary, all four corresponding dialkyl phosphoric acids ((n-C12H25O)(2)PO(OH), DDP; (n-C14H29O)(2)PO(OH), DTP; (n-C16H33O)(2)PO(OH), DHP; and (nC(18)H(37)O)(2)PO(OH) DOP) can form insoluble monolayers, with only the pi-A isotherm of DDP showing a phase transition plateau at 25degreesC. 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 x 10(-4) M in the subphases, pH similar to 5.6) on the pi-A isotherm of DDP, Ca2+ Sr2+, and Ba2+ (1 x 10(-4) M in the subphases, pH similar to 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 Ca2+, Sr2+, or Ba2+ 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.