A wave structure observed in the solar wind on 17 December 1990, is discussed, and it is found that this structure is associated with a slow-mode magnetosonic wave. The parameters of the wave such as the direction of the wave vector, the wave frequency and the decrement of attenuation, are estimated. It is shown that the peculiarities of the plasma motion in the wave may provide conditions corresponding to an effective adiabatic index less than unity. The variations of the proton density and temperature are analyzed. It proved that for the entire period of the wave structure passing the Earth, the value of the adiabatic index was less than unity, which agrees with the model by Pudovkin et al. [J. Geophys. Res. 102, 27145 (1997); Astron. Nachr. 320, 87 (1999)]. © 2000 American Institute of Physics.