Recent efforts on rational metabolic engineering approaches to increase butanol production in Clostridium acetobutylicum exhibited only limited success. Since multiple largely unknown parameters determine a particular phenotype, an inverse strategy to select a phenotype of interest can be useful. The major constraint for combinatorial metabolic engineering approaches is the availability of a feasible screening method. Therefore, a semi-quantitative assay to measure butanol and ethanol in cultures of C. acetobutylicum was developed. The applicability of the screening system was evaluated by two examples: First, C. acetobutylicum ATCC 824 was chemically mutagenized and subjected to high butanol concentrations as a pre-selection step. Screening of the population resulted in the identification of four mutants with increased butanol production as compared to the wildtype. The second application example was based on a C. acetobutylicum strain with low acetone biosynthetic activity, but concomitantly reduced butanol titer. After chemical mutagenesis, the population was analyzed and three mutants revealed significantly increased butanol concentrations in comparison to the parental strain. Thus, the suitability of the semi-quantitative screening system was validated, opening up new perspectives for explorative metabolic engineering of solventogenic clostridia.