The black liquor from the kraft pulping process is a potential source of organic acids (e.g. hydroxy acids) which could have applications as a raw material for biodegradable polymers. However, the recovery of the hydroxy acids has rarely been studied and they are not yet utilised. The aim of this paper is to study separation phenomena in the ultrafiltration and nanofiltration stages of the recovery and purification of organic acids from black liquor. Ultrafiltration, acidification, and cooling crystallisation were applied prior to purification by nanofiltration and adsorption.Direct ultrafiltration with a 1. kDa membrane removed 75% of the lignin. Acid molecules permeated the membrane faster than water molecules due to the Donnan exclusion and electroneutrality principles. As a result, the total acid concentration in the accumulated permeate stream was 1.4 fold compared to the original black liquor. However, ultrafiltration prior to precipitation and cooling crystallisation processes had a detrimental effect on the acid purity and the flux in downstream nanofiltration. The separation during nanofiltration in acidic conditions occurred mostly by size exclusion. It was also observed that the changes in the feed composition affected the retention of individual organic acids differently. Nanofiltration removed a significant amount of sulphate ions and residual phenolic compounds, but permeated most of the acid molecules in acidic pH conditions and finally the organic acid purity of the total dissolved solids in the NF permeate was about 80% at the highest.