The phonon spectra of Ga1−xInxAs have been studied by measuring the infrared reflectivity at near‐normal incidence in the 180 ∼400‐cm−1frequency range. Only one reststrahlen band corresponding to the GaAs‐like mode was observed forx≲0.2, and two bands were observed for the remaining composition. The data were analyzed by using the Kramers‐Kronig relationship and the dependence of the mode frequencies on alloy composition was interpreted on the basis of the modified cluster model including the effect of clusters. The fact that the calculated oscillator strength for the InAs‐like mode is substantially zero in the composition rangex≲0.2 explains the reason why phonons of the InAs‐like mode are not detected experimentally forx≲0.2. The clustering parameter representing the effect of clusters is determined by fitting the measured mode frequencies to the calculated compositional dependence and takes the maximum value near the midrange of alloy composition.