The JET Toroidal Field (TF) coils were originally designed for operation at 3.4 tesla. In order to upgrade the field to 4 tesla and thus improve the performance of the JET machine, new mechanical tests and analysis were carried out on the insulation of TF coil samples. They were aimed at investigating the mechanical properties and the status of the insulation in order to set allowable stresses and force limits. In particular since the shear stress in the insulation is strongly affected by the shear modulus of elasticity G, it is important to measure this parameter. A method for the measurement of G in glass–resin composites, the V-notched beam method (Iosipescu method), was applied. The particular shape of the rectangular Iosipescu V-notched sample and the particular modality of force application produce pure shear stress for a reliable measurement of the G value and of the shear strength of the insulation. The effect of temperature on these mechanical properties was also investigated. Results show higher average shear strength with lower scatter compared with previous tests on conventional rectangular samples, thus confirming the reliability of the method. Micrographic analysis of the insulation and comparison between the straight and curved regions of the magnet, where the highest stress occurs, confirm the good quality of the impregnation of the coil. Glass–resin content, void content, micros and TG measurements have been performed on different samples and correlation between the different properties of the insulation investigated. Moreover fatigue tests at different temperatures were performed and data analyzed with the cumulative damage technique, which allows for an extrapolation of the fatigue curve with less samples than the standard method.