Polymer gels are effective tools that are still widely used in mature oilfield development to stop unwanted fluid production from oil and gas wells, but conventional gelant formulations have become increasingly difficult to apply at low and ultralow temperatures. Because of this situation, the gelation performance of phenol–formaldehyde-based gel systems at a low temperature of 25°C is discussed in this article. The results show that the gelation time and strength of the gel systems can be perfectly controlled by the adjustment of the polymer concentration, the molecular weight, the crosslinking agent concentration, the ammonium salt concentration, and the composition. The polymer concentration and molecular weight can affect not only the gelation time and the gel strength but also its stability. The ammonium salt concentration affected not only the gelation time but also its viscosity before a detectable gel formed. Among them, the polymer concentration was the most important factor affecting the gel stability. For low-temperature reservoirs, the phenol–formaldehyde-based gel system achieved a much longer gelation time. Polymer gels formulated with a combination of 0.2–0.4 wt % polymer, 0.5–1.0 wt % formaldehyde or phenol–formaldehyde, and 0.1–0.6 wt % ammonium salt, and we added 0.02–0.03 wt % resorcinol to provide a gelation time between 2 h and 2 days. The maximum gel strength reached code I. The results of this study suggest that the formaldehyde-based gel system could be used effectively in low-temperature reservoirs. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40657.