Process for continuous dissolution of a solid in a reaction medium comprising the steps of: a. providing a liquid by withdrawal of a portion of the reaction medium from a first reaction vessel, the reaction medium comprising at least one unsaturated carboxylic anhydride of general formula R-C -O-C -R in which R represents an unsaturated organic radical having 2 to 12 carbon atoms, at least one unsaturated carboxylic acid of general formula R-COOH in which R is as defined above, at least one aliphatic carboxylic anhydride and at least one corresponding aliphatic carboxylic acid, wherein R is optionally substituted with any desired number of halogen atoms or cyano groups; b. contacting the liquid provided in step a) with the solid in a second reaction vessel to form a solution of the solid, wherein the solid in the second reaction vessel is present in the form of a fixed bed which is traversed by the liquid; and c. recycling the solution formed in step b) into the first reaction vessel, wherein the portion of the reaction medium withdrawn in step a) consists of aliphatic carboxylic acids to an extent of at least 90% by weight, the temperature of the fixed bed is set to a range from 10°C to 80°C, and the concentration of the solid in the solution obtained in step b) is measured continuously by means of a UV/VIS spectrometer, wherein the solid is a polymerisation inhibitor selected from the group consisting of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, phenothiazine, hydroquinone, hydroquinone monomethyl ether, 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl (TEMPOL), para-substituted phenylenediamines such as for example N,N'-diphenyl-p-phenylenediamine, 1,4-benzoquinone, 2,6-di-tert-butyl-alpha-(dimethylamino)-p-cresol, 2,5-di-tert-butylhydroquinone or mixtures of two or more of these stabilizers. a. providing a liquid by withdrawal of a portion of the reaction medium from a first reaction vessel, the reaction medium comprising at least one unsaturated carboxylic anhydride of general formula R-C -O-C -R in which R represents an unsaturated organic radical having 2 to 12 carbon atoms, at least one unsaturated carboxylic acid of general formula R-COOH in which R is as defined above, at least one aliphatic carboxylic anhydride and at least one corresponding aliphatic carboxylic acid, wherein R is optionally substituted with any desired number of halogen atoms or cyano groups; b. contacting the liquid provided in step a) with the solid in a second reaction vessel to form a solution of the solid, wherein the solid in the second reaction vessel is present in the form of a fixed bed which is traversed by the liquid; and c. recycling the solution formed in step b) into the first reaction vessel, wherein the portion of the reaction medium withdrawn in step a) consists of aliphatic carboxylic acids to an extent of at least 90% by weight, the temperature of the fixed bed is set to a range from 10°C to 80°C, and the concentration of the solid in the solution obtained in step b) is measured continuously by means of a UV/VIS spectrometer, wherein the solid is a polymerisation inhibitor selected from the group consisting of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, phenothiazine, hydroquinone, hydroquinone monomethyl ether, 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl (TEMPOL), para-substituted phenylenediamines such as for example N,N'-diphenyl-p-phenylenediamine, 1,4-benzoquinone, 2,6-di-tert-butyl-alpha-(dimethylamino)-p-cresol, 2,5-di-tert-butylhydroquinone or mixtures of two or more of these stabilizers. the portion of the reaction medium withdrawn in step a) consists of aliphatic carboxylic acids to an extent of at least 90% by weight, the temperature of the fixed bed is set to a range from 10°C to 80°C, and the concentration of the solid in the solution obtained in step b) is measured continuously by means of a UV/VIS spectrometer, wherein the solid is a polymerisation inhibitor selected from the group consisting of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, phenothiazine, hydroquinone, hydroquinone monomethyl ether, 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl (TEMPOL), para-substituted phenylenediamines such as for example N,N'-diphenyl-p-phenylenediamine, 1,4-benzoquinone, 2,6-di-tert-butyl-alpha-(dimethylamino)-p-cresol, 2,5-di-tert-butylhydroquinone or mixtures of two or more of these stabilizers.