Process for preparing a product mixture by industrial hydroformylation of an isobutene-containing hydrocarbon stream and for separating the resulting product mixture, comprising the following steps: a) hydroformylating the isobutene-containing hydrocarbon stream in a hydroformylation reactor in the presence of a transition metal complex catalyst, so as to obtain a product mixture comprising at least 3-methylbutanal, conversion products in the form of high boilers and 3-methylbutanoic acid, and the transition metal complex catalyst along with the free ligands thereof, b) separating the product mixture by means of a nanofiltration device comprising one or more membrane separation stages, such that the transition metal complex catalyst and the free ligands thereof are enriched in the resulting retentate of the nanofiltration device with respect to 3-methylbutanal and 3-methylbutanoic acid, and such that 3-methylbutanal and 3-methylbutanoic acid are each enriched in the resulting permeate of the nanofiltration device with respect to the transition metal complex catalyst, the concentration of the 3-methylbutanoic acid being lower in the resulting retentate than in the permeate, c) separating the resulting permeate of the nanofiltration device by means of a thermal separating device comprising one or more separation stages into at least one first fraction and a second fraction, the first fraction having a higher concentration of 3-methylbutanal than the second fraction and a lower concentration of conversion products in the form of high boilers and 3-methylbutanoic acid than the second fraction, d) recycling at least a substream of the resulting retentate of the nanofiltration device into the hydroformylation reactor; and further comprising the step of: - monitoring the concentration of 3-methylbutanoic acid in the resulting retentate of the nanofiltration device, and preferably also in the product mixture and/or in the resulting permeate of the nanofiltration device. a) hydroformylating the isobutene-containing hydrocarbon stream in a hydroformylation reactor in the presence of a transition metal complex catalyst, so as to obtain a product mixture comprising at least 3-methylbutanal, conversion products in the form of high boilers and 3-methylbutanoic acid, and the transition metal complex catalyst along with the free ligands thereof, b) separating the product mixture by means of a nanofiltration device comprising one or more membrane separation stages, such that the transition metal complex catalyst and the free ligands thereof are enriched in the resulting retentate of the nanofiltration device with respect to 3-methylbutanal and 3-methylbutanoic acid, and such that 3-methylbutanal and 3-methylbutanoic acid are each enriched in the resulting permeate of the nanofiltration device with respect to the transition metal complex catalyst, the concentration of the 3-methylbutanoic acid being lower in the resulting retentate than in the permeate, c) separating the resulting permeate of the nanofiltration device by means of a thermal separating device comprising one or more separation stages into at least one first fraction and a second fraction, the first fraction having a higher concentration of 3-methylbutanal than the second fraction and a lower concentration of conversion products in the form of high boilers and 3-methylbutanoic acid than the second fraction, d) recycling at least a substream of the resulting retentate of the nanofiltration device into the hydroformylation reactor; - monitoring the concentration of 3-methylbutanoic acid in the resulting retentate of the nanofiltration device, and preferably also in the product mixture and/or in the resulting permeate of the nanofiltration device.