Methylcelluloses are widely used for industrial purposes due to their peculiar properties. These are the consequence of associative behaviour due to hydrophobic interactions involving highly substituted units. The properties of commercial samples with heterogeneous distribution of the substituents and laboratory-made samples, with a more homogeneous distribution, are compared to allow a better understanding of the thermogelation process of methylcelluloses. Whatever the structure (degree of substitution, repartition of substituents and presence or not of highly substituted units) the same steps occur during this process, but the physicochemical observations (critical temperatures of physical changes for example) depend upon this structure.