Gel formation characteristic of dilute phenol-formaldehyde solution in water is studied at an elevated temperature near 140 degrees C. It is experimentally found that at a temperature range of 135 degrees C to 140 degrees C, a solution containing 3.0% phenol with the phenol-formaldehyde mole ratio 1.0:3.5 forms a gel in the pH range of 9.6 to 12.0. The minimum gelling time is observed at a pH value of 10.4. The gel formation is found to be strongly dependent on the pH at this elevated temperature. An explanation for the gelling behaviour is presented, which is based on an optimum ion concentration of Na+ to initiate the tri-methylol formation. It is hypothesized that the gel formation begins when concentration of tri-methylol phenol attains a threshold concentration value. Experimental observations and a kinetic analysis provide support for this hypothesis.