A comprehensive and comparative experimental study has been conducted to investigate the effect of thermal-oil ageing on mechanical, thermal and internal structural properties of E-glass fibre/epoxy composite materials. E-glass fibre/epoxy composite specimens were divided into 8 groups according to the certain experimental ageing temperatures. The temperature values were selected as -10, 25, 50, 80, 100, 120 and 140 degrees C, while the values of the ageing duration were selected as 24, 168, 360, 720 and 1080 h. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) tests were performed to determine the thermal properties, tensile tests were used to detect the mechanical properties and a Scanning Electron Microscope (SEM) was used to illustrate the changes in the internal structure of the composites. As a result of this work, unexpectedly no alteration was observed in the mechanical properties of the specimens aged at -10 degrees C; however, ageing at 120 and 140 degrees C reduced the maximum load carrying capacities (LCCs) of the specimens especially for further ageing exposure times. Furthermore, the thermal stability of the specimens aged at -10 degrees C increased with the increasing thermal ageing duration while, for the other thermal ageing temperatures, the thermal stability showed a decreasing trend as the thermal ageing exposure time increased.