The system Cu2+/H2O3P-C2H4-SO3H/NaOH was investigated using in situ energy dispersive X-ray diffraction (EDXRD) to study the formation and temperature induced phase transformation of previously described copper phosphonosulfonates. Thus, the formation of [Cu-2(O3P-C2H4-SO3)(OH)(H2O)]center dot 3H(2)O (4) at 90 degrees C is shown to proceed via a previously unknown intermediate [Cu-2(O3P-C2H4-SO3)(OH)(H2O)]center dot 4H(2)O (6), which could be structurally characterized from high resolution powder diffraction data. Increase of the,reaction temperature to 150 degrees C led to a rapid phase transformation to [Cu-2(O3P-C2H4-SO3)(OH)(H2O)]center dot H2O (1), which was also studied by in situ EDXRD. The comparison of the structures of 1, 4, and 6 allowed us to establish a possible reaction mechanism. In addition to the in situ crystallization studies, microwave assisted heating for the synthesis of the copper phosphonosulfonates was employed, which allowed the growth of larger crystals of [NaCu(O3P-C2H4-SO3)(H2O)(2)] (5) suitable for single crystal X-ray diffraction. Through the combination of force field calculations and Rietveld refinement we were able to determine the crystal structure of [Cu-1.5(O3P-C2H4-SO3)] 2H(2)O (3) and thus structurally characterize all compounds known up to now in this well investigated system. With the additional structural data we are now able to describe the influence of the pH on the structure formation.