The present invention relates to the formation of a gel within a medium M, in particular within an underground formation, which comprises the following steps: (E1) a basic compound C is added to a fluid latex L comprising dispersed particles of a polymer P, bearing ester side functions, said basic compound C being added in an amount suitable for hydrolysing at least one part of said ester side functions into carboxylic acid and/or alcohol functions, as a result of which a modified latex L' is obtained, the viscosity of which gradually increases with time from the addition of the compound C up to the formation of a gel; then (E2) the resulting modified latex is introduced into the medium M before the formation of the gel and the gel is left to form within the medium M.