LaMn1-xMgxO3 perovskite catalysts (x=0-0.5) were synthesised by the so-called "citrates method", characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO3 and LaMn0.8Mg0.2O3). The major results obtained are: (i) Mg substitution in the basic LaMnO3 perovskite has a positive effect on the catalytic activity only at low x values (x less than or equal to 0.2); (ii) as opposed to the results of previous studies on the LaCr1-xMgxO3 system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800-1200 degrees C) and on the value of x; (iii) an Eley-Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO3, the catalytic combustion over LaMn0.8Mg0.2O3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.