We studied the pore structure of activated carbons (ACs) formed during the thermolysis of Aleksandriya brown coal (Ukraine), which was preliminarily impregnated with alkali metal hydroxides, МОН with M being Li, Na, or K. We explored how pore structure parameters such as surface area, a total pore volume, meso- and micropore volume are modified with changing processing parameters such as temperature, an alkali/coal ratio (up to 2 g/g) and the nature of alkali cation. We also determined pore size distribution for meso- and micropores and compared all the three hydroxides as to their effect on porosity development. The formation of microporous structure is evidenced on heating a KOH-impregnated coal (1 g/g) from 400 to 800 ⁰С: a surface area rises from 13 to 1005 m2/g, while a micropore volume fraction increases from 1 to 68% of the total pore volume. Microporous carbon is formed at 800 ⁰C over 1 h in a yield of 30% with S_BET=1005 m2/g, a total pore volume of 0.55 cc/g and a micropore volume of 0.38 cc/g. The total pore volume grows monotonously in the range LiОН