The effect of four different phenolic compounds (i.e., phenol, catechol, resorcinol, and hydroquinone) on the performance of organically cross-linked terpolymer gel systems at the temperature of 150 degrees C was investigated. The phenol-based gelant systems were not able to form visible bulk gels at this extremely high temperature because the cross-linked clusters between phenol and hexamethylenetetramine (HMTA) only contained a small number of hydroxyl groups for cross-linking reactions. The catechol- and hydroquinone-based gelants were able to form relatively strong bulk gels because the amount of the cross-linked clusters between these two phenolic compounds and HMTA increased significantly. This increment also contributed to the decrease of the grid sizes of the gel network structures and the emergence of dendritic structures on them, thereby significantly increasing the viscosity, storage modulus, and thermal stability of the obtained gels. However, these two gel systems could not be maintained for long; syneresis began after only 3-12 days of the systems being held at 150 degrees C. When phenol was replaced by resorcinol, bulk gels with excellent strength and long-term thermal stability were able to form at 150 degrees C. The use of the gelation mechanism of the cross-linking reactions between the terpolymer and different cross-linker systems can help researchers and petroleum engineers better understand the differences between the different cross-linker systems and thus develop more suitable polymer gel systems for water management in extremely high temperature reservoirs.