The temperature dependence of the elastic functions of the improper ferroelastic polytype 2MC-Sb5O7I has been investigated in the temperature range from room temperature to well above the structural phase transition atT c=481 K. The stiffnessesc(c*c*),c(a,a),c 22 andc(c*a) show a considerable softening up to 20% aroundT c whereasc 44 remains unaffected by the phase transition. The experimental results are discussed considering cubic and quartic anharmonic coupling between two components of the zone boundary order parameter and elastic waves.