A process for recovering diphosphonite-containing compounds from a feed mixture comprising diphosphonite-containing compounds, organic mononitriles, organic dinitriles and a Lewis acid in a multistage countercurrent liquid-liquid extractor with extraction solvent comprising aliphatic hydrocarbon, cycloaliphatic hydrocarbon or a mixture of aliphatic and cycloaliphatic hydrocarbon, said process comprising: a) flowing the feed mixture to a mixing section of a mixer-settler in the first stage of the multistage countercurrent liquid-liquid extractor; b) contacting the feed mixture with extraction solvent from the second stage of the multistage countercurrent liquid-liquid extractor in the mixing section of the mixer-settler of the first stage to form a mixed phase comprising a light phase and a heavy phase, wherein the light phase comprises extraction solvent and extracted diphosphonite-containing compounds, and wherein the heavy phase comprises organic mononitrile and organic dinitriles; c) passing the mixture from step (b) to a settling section in the mixer-settler of the first stage, wherein the mixed phase separates into the light phase and the heavy phase; and d) characterized by introducing a Lewis base into the settling section of the first stage, wherein a complex of Lewis acid and Lewis base is formed in the settling section in the mixer-settler of the first stage, wherein at least a portion of the light phase is withdrawn from the settling section and treated to recover diphosphonite-containing compounds extracted into the light phase, and wherein at least a portion of the heavy phase is passed to the second stage of the multistage countercurrent liquid-liquid extractor; wherein the Lewis base is selected from the group consisting of: a) polyamine; and b) poly(acrylic acid), sodium salt. a) flowing the feed mixture to a mixing section of a mixer-settler in the first stage of the multistage countercurrent liquid-liquid extractor; b) contacting the feed mixture with extraction solvent from the second stage of the multistage countercurrent liquid-liquid extractor in the mixing section of the mixer-settler of the first stage to form a mixed phase comprising a light phase and a heavy phase, wherein the light phase comprises extraction solvent and extracted diphosphonite-containing compounds, and wherein the heavy phase comprises organic mononitrile and organic dinitriles; c) passing the mixture from step (b) to a settling section in the mixer-settler of the first stage, wherein the mixed phase separates into the light phase and the heavy phase; and d) characterized by introducing a Lewis base into the settling section of the first stage, wherein a complex of Lewis acid and Lewis base is formed in the settling section in the mixer-settler of the first stage, wherein at least a portion of the light phase is withdrawn from the settling section and treated to recover diphosphonite-containing compounds extracted into the light phase, and wherein at least a portion of the heavy phase is passed to the second stage of the multistage countercurrent liquid-liquid extractor; wherein the Lewis base is selected from the group consisting of: a) polyamine; and b) poly(acrylic acid), sodium salt. a) polyamine; and b) poly(acrylic acid), sodium salt.