The specific heats of the semiconducting compounds ZrSe 3 and ZrS 3 have been measured in the temperature range between 8 and 200 K. At the lowest values of T, the ZrSe 3 measurements can be analyzed in terms of the familiar Debye T 3 law yielding a characteristic Debye temperature at absolute zero θ D (0) = 110 ± 2 K in satisfactory agreement with the value extracted from the available experimental sound velocities. In contrast, the low-temperature ZrS 3 measurements do not behave according to the Debye law. This is interpreted as resulting from a strong mixing of low-lying optical modes with the acoustical branches, and reflects a more pronounced one-dimensional character of ZrS 3 compared to ZrSe 3.