Modulus of elasticity of loosely cross-linked poly(isobutylene) gels are measured under large compressive deformations. Gels of perfect spherical shape are prepared by the suspension cross-linking technique of dilute toluene solutions of butyl rubber using sulfur monochloride as a cross-linking agent. Compressive deformation is applied by a calibrated stainless steel cantilever to spherical poly(isobutylene) beads of sizes ca. 1.0-4.0 mm in diameter swollen in toluene. The relationship between force and deformation is obtained by using the constitutive relations from molecular theory of elasticity Moduli are measured immediately after applying compression, and after full relaxation which obtains after solvent exudes the gels under compression. The instantaneous moduli are an order of magnitude larger than equilibrium moduli. Results of mechanical measurements, which are presently of a preliminary type, are compared with those from independent thermodynamic estimates of shear moduli. (C) 2001 Elsevier Science Ltd. All rights reserved.