A slurry of monoaluminum phosphate with fine Al2O3 powder was infiltrated into eight harness satin fabric of Nicalon(TM) fibres. The infiltrated fabrics were laid up in 16 plies and then cured in an autoclave. Due to the chemical reaction between alumina and monoaluminum phosphate, aluminum phosphate was produced. Carbon coated and uncoated fibres were used to manufacture these composites to produce weak and strong interfaces between matrix and fibres. Two different processing routes were used during manufacturing, which resulted in different amounts of porosity content in the final composites. Thermogravimetric analysis carried out at 850-degrees-C for 6000 min in air showed negligible weight change, indicating stable fibres in the composites. Four-point bend flexure tests were conducted at room temperature, 700 and 850-degrees-C. Strong interface composites completely failed at high temperatures due to fibre fracture, but a shear dominated delamination-type interface failure was observed for the weak interface composites. Strong interface composites exhibited better flexure strengths than the weak interface composites under all conditions of processing and orientations. However, the weak interface composites absorbed significantly larger amounts of energy before failure compared to the strong interface composites. Little influence of porosity content was seen on flexure properties both at room and elevated temperatures. High temperature failure behaviour is explained using classical lamination theory.