Lignin was isolated from Cajanus cajan (Pigeon pea; Bengali : arhar). Lignosulphonic acid resin was prepared from this lignin and characterised by pH titration. This lignosulphonic acid showed titration characteristics resembling those of soluble strong acids. Addition of electrolytes to water increased the ion-exchange capacity. Sorption of moisture by lignosulphonic acid showed a gradual moisture gain upto about 80% relative humiduty which was followed by a sharp increase of moisturegam upto 100% relative humidity.