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This study investigates the crucial role of sodium sulfate in pulp washing liquors and its impact on the reattachment of lignin to pulp fibres during the brownstock washing process. The dissolution of lignin during washing and its potential redeposition onto the pulp fibres is influenced by various factors. Three distinct types of pulp – unbleached, bleached, and cotton linters – were used to explore these effects. The washing experiments were conducted using industrial wash liquor and were repeated further with the addition of sodium sulfate. The resulting products of the washing process, including the liquor discharge and the washed pulp, were thoroughly evaluated. Analytical techniques, such as UV measurements of lignin content in the liquor discharge and characterization of the pulp, were employed to assess the outcomes. The findings reveal that the addition of sodium sulfate to the washing liquor results in an increase in its conductivity and ionic strength. Moreover, it was observed that lignin reattachment to pulp fibres was significantly greater when washing was performed with sodium sulfate addition. Among the pulp studied, unbleached kraft pulp exhibited the highest degree of lignin reattachment, followed by bleached kraft pulp, with cotton linters showing the least.