Good firmness is an essential property of acceptable cigarette filters. Unfortunately, good firmness is not always consistent with the desired pressure drop, weight, and filter performance. The factors which affect filter firmness as perceived by the smoker were studied by measuring the TEC hardness of a set of systematically selected filter rods after complete curing had taken place. The effects of filter specifications on filter firmness were quantitatively described by using the results of these experiments, and predictive models were derived. The filter tow parameters studied were denier per filament, total denier, and fiber cross section. The filter rod variables investigated include retained crimp ratio, circumference, plasticizer level, and plug wrap type. In addition, filter firmness was studied as a function of the force applied by the hardness tester, and the resulting information was used to derive an equation that relates TEC and Filtrona hardness values . The qualitative results of the study are that filter firmness is improved by increasing plasticizer levels, denier per filament, total denier, and retained crimp ratio or by decreasing rod circumference. However, because all these factors are interactive, it is not always obvious how firmness will change when several filter specifications are changed simultaneously; the use of the quantitative, predictive model derived in this study is demonstrated for determining firmness changes in these cases.