The 2013 FECOND (Fertility, Contraception, and Sexual Dysfunction) probability telephone survey aims to monitor sexual health behaviors among fifteen to forty-nine year olds in France. We conducted a random experiment to compare a classic telephone survey (group T, n = 3,846 respondents) with two Internet-telephone mixed-mode protocols: a sequential Internet-telephone protocol (group S, n = 762, among which there were 462 Internet questionnaires), and a concurrent protocol (group C, n = 1,165, among which there were 208 Internet questionnaires). We compare telephone (T), sequential (S), and concurrent (C) samples on cooperation rates, break-off, and item nonresponse rates, sociodemographic characteristics, health behaviors, and seven sexual health behaviors and personal opinions questions. Reports on the most sensitive behaviors were expected to be more truthful and more prevalent on the Internet—and thus in the mixed-mode samples—than in the telephone sample. The cooperation rate (i.e., the response rate among the possible respondents selected during the initial telephone call) was higher in the classic telephone survey than in the sequential and concurrent mixed-mode protocols (88 percent for T versus 77 percent for S and 55 percent for C), where break-off and item nonresponse rates were also higher. Despite these lower response rates, mixed-mode samples showed better representativeness: their marginal distribution of sociodemographic characteristics was closer to that of the 2013 census, and they had higher R-indicators. A causal estimation of the measurement effect resulting from Internet administration found higher prevalence of three out of the seven sexual health behaviors and personal opinions in the sequential protocol compared to the classic telephone group; a similar pattern was found in the concurrent protocol. In addition, the variance of the weights of the mixed-mode protocols is lower, especially for the sequential design. Sequential telephone-Internet mixed-mode protocols nested in a probability telephone survey may be a good way to improve survey research on sensitive behaviors.