Ln-dpp-DO3A-based complexes [dpp is a pendant diphenylphosphinamide moiety and DO3A is 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane] exhibit pH-responsive reversible ligation of the phosphinamide for both the gadolinium(III) and europium(III) analogues. pK(a) values were 8.1 (+/- 0.1) and 7.8 (+/- 0.1) for Gd.1 and Gd.2, respectively. The relaxivities (20 MHz, 298 K) of the gadolinium(III) analogues were r(1) = 7.9 mM(-1) s(-1) (Gd.1) and r(1) = 8.2 mM(-1) s(-1) (Gd.2) in acidic media, corresponding to a hydration state q = 2; in basic media, deprotonation and coordination of the phosphinamide occurs, with r(1) = 5.4 mM(-1) s(-1) and r(1) = 4.4 mM(-1) s(-1) (Gd.2) corresponding to q = 1. Sensitized luminescent emission was observed from the europium(III) analogues following excitation at lambda(ex) = 270 nm. The hydration state values of the europium(III) analogues were consistent with those of the gadolinium(III) complexes, i.e., q = 1 and 2 in basic and acidic media, respectively. The ratio of the emission intensities of the Delta J = 1 and 2 bands enables concentration-independent reporting of the pH. Excited-state pK(a) values were 8.3 (+/- 0.1) and 8.5 (+/- 0.1) for Eu.1 and Eu.2, respectively.