The efficiency of thermal shock (TS), which was introduced instead of temperature-programmed heating (4 K/min) in the alkaline activation (KOH, 800°C) of fossil coals of the entire metamorphic series (Сdaf = 70.4–95.6%), was evaluated by comparing the porous structure characteristics of activated carbons (ACs). ACs with increased specific surface areas (by a factor of ≤2.5) and pore volumes (by a factor of ≤2.2) but with lower (by a factor of 1.07–1.30) yields were formed upon TS. The effect of TS expressed by an increase in the specific surface area ∆S was maximal on the conversion of brown coal (∆S = 870 m2/g), and it decreased to a minimum in coals with Сdaf = 90 ± 1% (∆S = 109 m2/g) or increased on the activation of anthracites (∆S = 496 m2/g). It was shown that the efficiency of TS increased with Odaf, which is responsible for the formation of subnanopores, for lignites and coals. On the activation of anthracites, an additional pore formation mechanism not associated with oxygen groups was activated.