Abstract In this paper mathematical models of a conventional and modified passive suspension of working machine seats are presented. A seat with a viscous-elastic passive suspension, whose vibro-isolation properties are improved by a modification of an air-spring and shock absorber, is the object of modelling and simulation. For a vibration isolating system defined in such a way, a set of poly-optimal solutions was determined using two opposing criteria: the acceleration of the vehicle operator and the relative displacement of seat suspension. The power spectral density of the acceleration of the seat with respect to sample excitations on the working machine floor are presented. Transmissibility functions for a passive conventional suspension and the corresponding modified seat are also presented. The modified seat showed improved performance over the conventional seat in the 0–4 Hz frequency range.