It is important to examine the long-term durability of glass-kenaf fibre reinforced phenolic resin composites when they are exposed to humid environments or submerged in water. Furthermore, the durability of such composites when immersed in different pH solutions have yet to be examined. As such, this present study examined the use of 4%, 8%, and 12% volume fractions (vfs) of microcrystalline cellulose (MCC) as a filler and reinforcement to improve the properties of glass fibre-kenaf reinforced phenolic resin composites. The flexural strength of these composites was examined both pre- and post-immersion in distilled water (pH 7), seawater (pH 8), and an acidic solution (pH 3) for 60 days. The diffusion mechanism, difussion coefficient, and water absorption concentration were also examined. The difussion coefficient and water absorption concentration occurred post-immersion in distilled water (pH7) and seawater (pH8) while the acidic solution (pH3) resulted in the highest loss of mass and size. Scanning electron microscopy (SEM) of the surfaces of the saturated composites indicated that fibre-matrix interfacial bonding was weak. However, composites that contained a higher vf of MCC exhibited stronger interfacial bonding between the matrix and constituents, thereby, reducing water absorption and diffusion. The flexural strength of the composite pre- and post-immersion was MCC12 > MCC8 > MCC4 > MCC0, in descending order of strength.