The present article provides a novel technique for the co-amorphous formation of acyclovir and oxalic acid as an excipient. Designated as ACV-oxalic acid, methods for the preparation thereof and its use in pharmaceutical applications are described. The co-amorphous ACV-oxalic acid was characterized by powder X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis. Stability with respect to relative humidity (RH) for the co-amorphous ACV-oxalic acid was evaluated and compared with the parent ACV. The aqueous solubility of the co-amorphous ACV-oxalic acid was significantly improved (about 8 times more soluble) at 35°C compared to that of the parent Acyclovir base.