  Perturbations in a Chaplygin gas, characterized by an equation of state $p =
-A/\rho$, may acquire non-adiabatic contributions if spatial variations of the
parameter $A$ are admitted. This feature is shown to be related to a specific
internal structure of the Chaplygin gas. We investigate how perturbations of
this type modify the adiabatic sound speed and influence the time dependence of
the gravitational potential which gives rise to the Integrated Sachs-Wolfe
effect in the anisotropy spectrum of the cosmic microwave background.
