Process for preparing a polyester carbonate comprising the steps of: (i) reacting a mixture comprising at least one linear aliphatic dicarboxylic acid and/or at least one cycloaliphatic dicarboxylic acid and at least one aliphatic and/or aromatic carbonate, in the presence of at least one first catalyst that is basic, to form an aliphatic diester of formula in which A in each case independently represents an aliphatic or aromatic radical, D represents R 3 or one of formulas or, where R 3 represents a linear alkylene group having 3 to 16 carbon atoms and this alkylene group may optionally be mono- or polysubstituted or in which B in each case independently represents a CH 2 group, O or S, R 1 in each case independently represents a single bond or an alkylene group having 1 to 10 carbon atoms and R 2 in each case independently represents an alkylene group having 1 to 10 carbon atoms, n is a number between 0 and 3, m is a number between 0 and 6 and "*" indicate the positions at which the -(C=O)OA groups in formula are present, (ii) separating the aliphatic diester of formula from the mixture from process step (i), (iii) reacting the separated aliphatic diester of formula, at least one dihydroxy compound, and at least one diaryl carbonate in a melt transesterification process in the presence of a mixture comprising a second catalyst and a third catalyst, wherein the second catalyst is a tertiary nitrogen base, wherein the third catalyst is a basic alkali metal salt, and wherein the proportion of alkali metal cations in process step (iii) is 0.0010% to 0.0030% by weight based on all components used in process step (iii). (i) reacting a mixture comprising at least one linear aliphatic dicarboxylic acid and/or at least one cycloaliphatic dicarboxylic acid and at least one aliphatic and/or aromatic carbonate, in the presence of at least one first catalyst that is basic, to form an aliphatic diester of formula in which A in each case independently represents an aliphatic or aromatic radical, D represents R 3 or one of formulas or, where R 3 represents a linear alkylene group having 3 to 16 carbon atoms and this alkylene group may optionally be mono- or polysubstituted or in which B in each case independently represents a CH 2 group, O or S, R 1 in each case independently represents a single bond or an alkylene group having 1 to 10 carbon atoms and R 2 in each case independently represents an alkylene group having 1 to 10 carbon atoms, n is a number between 0 and 3, m is a number between 0 and 6 and "*" indicate the positions at which the -(C=O)OA groups in formula are present, A in each case independently represents an aliphatic or aromatic radical, D represents R 3 or one of formulas or, where R 3 represents a linear alkylene group having 3 to 16 carbon atoms and this alkylene group may optionally be mono- or polysubstituted or in which B in each case independently represents a CH 2 group, O or S, R 1 in each case independently represents a single bond or an alkylene group having 1 to 10 carbon atoms and R 2 in each case independently represents an alkylene group having 1 to 10 carbon atoms, n is a number between 0 and 3, m is a number between 0 and 6 and "*" indicate the positions at which the -(C=O)OA groups in formula are present, B in each case independently represents a CH 2 group, O or S, R 1 in each case independently represents a single bond or an alkylene group having 1 to 10 carbon atoms and R 2 in each case independently represents an alkylene group having 1 to 10 carbon atoms, n is a number between 0 and 3, m is a number between 0 and 6 and "*" indicate the positions at which the -(C=O)OA groups in formula are present, (ii) separating the aliphatic diester of formula from the mixture from process step (i), (iii) reacting the separated aliphatic diester of formula, at least one dihydroxy compound, and at least one diaryl carbonate in a melt transesterification process in the presence of a mixture comprising a second catalyst and a third catalyst, wherein the second catalyst is a tertiary nitrogen base, wherein the third catalyst is a basic alkali metal salt, and wherein the proportion of alkali metal cations in process step (iii) is 0.0010% to 0.0030% by weight based on all components used in process step (iii). wherein the second catalyst is a tertiary nitrogen base, wherein the third catalyst is a basic alkali metal salt, and wherein the proportion of alkali metal cations in process step (iii) is 0.0010% to 0.0030% by weight based on all components used in process step (iii).