The aim of this study is to determine differences in postoperative air-bone gap (ABG) after placement of teflon-platinum or nitinol middle ear prostheses in primary stapedotomy patients with otosclerosis. Thirty otosclerosis patients (24 female, 6 male; age 10-61 years) with primary stapedotomy were studied prospectively. Before and after surgery, the mean and standard deviations of the ABG were measured at eight frequencies (0.25-4 kHz). Patients were randomized into one of two groups receiving either teflon-platinum or nitinol prostheses. Hearing results were assessed 1 year after surgery. To assess the joint influence of treatment and frequency on ABG reduction, a linear mixed model was used (significance level was p = 5%). The Tukey-Kramer method was used to adjust for multiple comparisons. Significant differences were found between treatment groups (p < 0.0001) and between frequencies within the same treatment group (p < 0.0001) but no interaction (p = 0.7963), i.e. the reduction of the conductive components over frequencies was nearly parallel in both groups. Overall, patients in the Teflon group had a larger reduction of conductive components, on average 8.0 dB more reduction, than patients in the nitinol group. However, after adjusting for multiple comparisons, we could not identify a single frequency with a significant difference in reduction of conductive components. Use of the teflon-platinum prosthesis results in statistically non-significant better ABG closure at 0.25-4 kHz 1 year postoperatively than the use of the nitinol prosthesis.