1 ) Method for estimation of the quantity of fresh air (m aira, m airs ) present in the intake and exhaust manifolds ( 3, 5 ) of an internal combustion engine ( 1 ) with a recirculation circuit ( 9, 10 ), by an observer (O), applied to a system (S), which models the physical system constituted by the intake and exhaust manifolds ( 3, 5 ); the system (S) to be observed being obtained from differential equations, which regulate the processes of mixing of the gases, and from an equation which represents the behaviour of a lambda sensor ( 13 ).