In the 1920s and 30s Jämsänkoski pulp production was expanded and developed. The reforms were initiated by a fire in 1926 which damaged the factory so badly that in practice it had to be rebuilt. The aim was to increase production of pulp for sale. The old boilers were replaced with three 165 cubic metre boilers. The pulp containers were replaced and other sections enlarged.

Pulp production was tripled in ten years. During the slump years of the early 1930s a new bleaching plant was completed, and Jämsänkoski started selling bleached pulp also to other factories. The cooking section moved over to so-called indirect cooking, where steam is not piped direct into the boiler but into a system of pipes outside the boiler (calorisator) which heated the cooking liquor. In indirect cooking, the pulp became harder and the yield was greater. Wet wood, even timber lifted straight out of water, could be used as raw material. Thus, the mill's large timber store on Oinaala field could be closed. The new production buildings also changed the visual look of the mill area into that of large-scale industry, with its landmarks of concrete acid towers and silos.

Imports of foreign sulphur needed by the factory dried up due to the war. United signed a contract with Outokumpu Oy on supplies of domestic sulphur pyrites to the Valkeakoski and Jämsänkoski mills. The Jämsänkoski sulphur pyrites plant was built in 1942 - 1943.

Due to shortages of fuel and low water supplies, the pulp mill was forced to stop production several times as the war continued. Even in 1945 - 1946 the mill stood idle for a few months, but in 1947 production reached sufficient levels for foreign exports to begin.