The variation in the compressive strength of concrete block masonry was studied at elevated temperatures. Small specimens known as wallettes were used to obtain the compressive strength under steady state conditions. Eighteen wallettes were made of lightweight concrete blocks and 1:1:5 mortar proportion. The target temperatures were 20ºC, 200ºC, 400ºC, 600ºC, 700ºC and 800ºC. Initially load-deflection relationships were determined from the experimental wallettes and later they were converted into stress-strain relationships. Although the goal was to determine the compressive strength, other parameters were also studied such as modulus of elasticity, temperature-time relationships, modes of failure, material degradation, and change of colour.Lightweight concrete blocks were also tested to determine the compressive strength at equal temperatures applied for the wallettes. The blocks belonged to the same batch used for the wallettes. The tensile behaviour of mortar was determined at 20ºC, 200ºC and 400ºC.Once the mechanical properties of the masonry wallettes, units and mortar were determined, they were used as input data to develop finite element models to simulate the same behaviour of the experimental wallettes. Finally, using the experimental and numerical results from the wallettes, they were used to predict the behaviour of 3m height walls.