Effects of environmental conditions on failure loads of multi-bolt tension joints in pultruded GRP (glass-reinforced plastic) plate have been quantified by physical testing. A simple bolt layout-two bolts on the tension axis-was selected. joint geometry was defined in terms of the end distance (E), the pitch distance (P), the side distance (S) and the bolt diameter (D). The joints were tested under four sets of environmental conditions: (a) normal (ambient temperature (similar to 20 degrees C) + dry), (b) hot (60 degrees C + dry), (c) wet (ambient temperature + 6.5 weeks in water), and (d) hot-wet (60 degrees C + 6.5 weeks in water). They were also tested with various geometries-that is, three, two and two values of the ratios (E/D), (P/D) and (S/D) respectively. A total of 144 joints were tested. Damage and failure loads together with failure modes were recorded. The effects of environmental conditions and joint geometries on damage and failure loads have been quantified and load reduction (knock-down) factors have been established for each of the environmental conditions.