  A generalized plasma model having warm ions, iso-thermal electrons,
super-thermal electrons and positrons is considered to theoretically
investigate the modulational instability (MI) of ion-acoustic waves (IAWs). A
standard nonlinear Schr\"{o}dinger equation is derived by applying reductive
perturbation method to study the MI of IAWs. It is observed that the MI
criteria of the IAWs are significantly modified by various plasma parameters.
The present results should be useful in understanding the conditions for MI of
IAWs which are relevant to both space and laboratory plasma system.
