After the war, the oldest part of the factory was refurbished in 1946 - 1948. The collection section, pulp chests and packing and loading areas were rebuilt. The hand-shovelling of pulp ended when emptying of the pulp containers was automated. In the early 1950s the bleaching plant was extended and bleaching became continuous rather than intermittent. The whole pulp output of the factory could now be bleached.

After Myllykoski separated from United Paper Mills Ltd, investments were concentrated on building a new paper mill - Kaipola. Updating of the Jämsänkoski pulp mill was not implemented until 1958 - 1966. Two new boilers were purchased and steam production increased. Emptying of the pulp containers was mechanised, and the premises of the sifting section and bleaching plant were extended. Two thirds of the pulp produced was used at the company's own factories, one third was exported. The Kaipola mill had been assumed to use a large part of Jämsänkoski pulp, but bleached sulphate pulp was more suitable for the printing paper it produced.

The pulp mill boiler room was extended in 1959 - 1960. The old pyrite furnace was replaced with equipment for burning molten elemental sulphur. The new 60-metre high acid towers were completed in 1963. A second-hand Kamyr machine was purchased for drying, and the unbleached pulp line started up in 1962. Halfway through the decade, production reached 79,000 tonnes. Because there was a surplus in the markets, sale of pulp to outsiders was discontinued in 1965.

In the 1950s the output of the pulp mill doubled, and in the 1960s growth was still at 50 percent. This was also evident in the condition of the waterway downstream. The most important investment at the Jämsänkoski pulp mill was construction of an evaporation and incineration plant for waste liquor in 1969. Although the plant was capable of treating the waste liquor to 95 percent, the environmental problems caused by the pulp mill were only resolved when the factory closed down in 1981. The environmental damage caused by the pulp mill is discussed in the section 'Environmental changes'.

By the 1970s the pulp mill was already an outdated plant. More than ten new sulphate cellulose factories had opened in Finland, while the old sulphite factories were run down. In addition to Jämsänkoski, the last mills were in Mänttä, Nokia, Valkeakoski, Äänekoski and Kemi. The old factories were given a reprieve by taking on ancillary production; manufacture of Pekilo protein and sulphite spirit. At the end of the decade, the fate of the Jämsänkoski works had to be decided. The result of an investigation was a change in production policy. Instead of pulp-based paper types, it was decided that the main raw material would be thermomechanical pulp developed by the Kaipola factory. The sulphite pulp mill ceased its operation in 1981.