A method for estimating the composition of LPG in an LPG supply system for engines with positive ignition, said LPG supply system comprising an LPG tank, a pressure reducer, a first conduit that extends between the tank and the pressure reducer, a plurality of injectors and a second conduit that extends between the pressure reducer and the injectors, said method comprising the steps of: - recognising an incipient condensation condition of the LPG, said incipient condensation condition of the LPG occurring when the LPG in vapour form starts to pass into liquid form; - in response to the recognition of said incipient condensation condition of the LPG, detecting first LPG injection pressure and temperature values; - starting from the first LPG injection pressure and temperature values detected, obtaining a corresponding percentage of propane and/or butane contained in the LPG, wherein the step of recognising the incipient condensation condition of the LPG comprises the steps of: - identifying a first series of parameters indicating a possible incipient condensation condition of the LPG, wherein the first series of parameters comprises the ratio between air and LPG and/or the temperature of the LPG; - identifying a second series of parameters associated with said first series of parameters, wherein the second series of parameters comprises one or more of the following: minimum engine load, engine load variation, engine revolutions, gas temperature, engine cooling liquid temperature, purging condition of a canister, diagnosis condition, switch-over condition, cut-off exit condition; - for each parameter of the second series defining an observation window for the parameters of the first series, said observation window being delimited upwards and/or downwards by threshold values assumed by said parameter of the second series; - detecting values of the parameters of the second series and verifying whether said values are within the corresponding observation windows; - detecting values of the parameters of the first series and, in response to the detected values of the parameters of the second series within the corresponding observation windows, checking whether the detected values for the parameters of the first series are compatible with an incipient condensation condition for the LPG. - recognising an incipient condensation condition of the LPG, said incipient condensation condition of the LPG occurring when the LPG in vapour form starts to pass into liquid form; - in response to the recognition of said incipient condensation condition of the LPG, detecting first LPG injection pressure and temperature values; - starting from the first LPG injection pressure and temperature values detected, obtaining a corresponding percentage of propane and/or butane contained in the LPG, - identifying a first series of parameters indicating a possible incipient condensation condition of the LPG, wherein the first series of parameters comprises the ratio between air and LPG and/or the temperature of the LPG; - identifying a second series of parameters associated with said first series of parameters, wherein the second series of parameters comprises one or more of the following: minimum engine load, engine load variation, engine revolutions, gas temperature, engine cooling liquid temperature, purging condition of a canister, diagnosis condition, switch-over condition, cut-off exit condition; - for each parameter of the second series defining an observation window for the parameters of the first series, said observation window being delimited upwards and/or downwards by threshold values assumed by said parameter of the second series; - detecting values of the parameters of the second series and verifying whether said values are within the corresponding observation windows; - detecting values of the parameters of the first series and, in response to the detected values of the parameters of the second series within the corresponding observation windows, checking whether the detected values for the parameters of the first series are compatible with an incipient condensation condition for the LPG.