The problem of the maturing of thermal mud has long been a subject of discussion by many workers, without any objective solution being found. The process consists of the interaction between a fluid component (mineral water) and a solid component (virgin mud). Maturing is an extremely complex phenomenon which includes not only processes peculiar to the interaction between these components but also a wide range of biochemical processes related to the growth of algae and the micro-organisms characteristic of thermal mud. These processes are influenced by the ecosystem in which maturing takes place. Maturing involves a complex set of modifications which can be summed up as: (1) modifications related to the virgin mud, (2) appearance of new chemical species or an increase in those already present, and (3) disappearance or decrease in chemical substances already present. In the present paper, maturing has been investigated using thermal analysis, reflectance spectroscopy. X-ray diffraction, gas, liquid and ionic chromatography, atomic absorption and nuclear magnetic resonance. Families of molecules and physico-chemical parameters have been analysed to evaluate modifications as a function of time, with the aim of identifying any trends in such modifications which might lead to equilibrium conditions which could be considered indicative of maturing.