Two aminophosphonic acid dimethylesters 1 and 3 bearing COOH functions attached to UV-VIS absorbing groups were hydrolysed by aqueous NaOH to yield the corresponding monomethylesters 2 and 4. The rates of hydrolysis were monitored by UV-VIS methods. The type of reaction observed is of pseudo first order with rate constants k1[1 mol−1s−1] of 1.92.10−6(1, 25°C). 4.96.10−5(1, 50°C) and 4.63.10−5(3, 50°C). Hydrolysis is sufficiently slow to enable the practically undisturbed determination of dissociation constants for 1-(4-carbxyphenyl)−1-(N-t. butyl)-aminomethylphosphonic acid dimethylester (1) and 1-(4-benzoic-acid)−1-(N-4-phenyl-azo-phenylene)-aminomethylphosphonic acid dimethyl ester (3).