To produce materials mimicking biological soft tissues available for finite-amplitude bioacoustics, the acoustic nonlinearity parameter B/A in a water--oil mixture at the sol and gel states has been investigated experimentally. The sol sample was supplied by ultrasonically emulsifying a mixture of rapeseed oil and water at various volume fractions. The gel was obtained by mixing gelatin powder to the warmed sol. The B/A was measured by utilizing the fundamental and second-harmonic components of a 1.9-MHz focused Gaussian beam, while the sound speed was measured by the sing-around method applied to the same focusing system [S. Saito, J. Acoust. Soc. Am. 93, 162--172 (1993)]. The measured data have been investigated through comparison with the previously proposed mixture rules [R. E. Apfel, J. Acoust. Soc. Am. 74, 1866--1868 (1983); C. M. Sehgal et al., Ultrasound Med. Biol. 12, 865--874 (1986)].