## Abstract

Some vibration-rotation bands of monodeuteroacetylene have been measured with high resolution. Values have been derived for the coefficients $\alpha _{i}$ relating the rotational constants in different vibrational levels, as follows: $\alpha _{2}$ = + 0$\cdot $00439, $\alpha _{3}$ = + 0$\cdot $00638, $\alpha _{4}$ = - 0$\cdot $0032$_{2}$, $\alpha _{5}$ = - 0$\cdot $0011. Using the value B$_{00000}$ = 0$\cdot $9910$_{5}$ cm$^{-1}$, also determined from many bands, a new value, B$_{e}$ = 0$\cdot $9948, has been obtained leading to new estimates for the bond lengths r$_{e}$CH = 1$\cdot $058 angstrom, and r$_{e}$C$\equiv $C = 1$\cdot $205$_{0}$. The l-doubling coefficient has been determined in two states, namely, q$_{00010}$ = 0$\cdot $0056 and q$_{00003}$ = 0$\cdot $0072. In the ground state the results are in accordance with a centrifugal stretching coefficient D = 0$\cdot $7 $\times $ 10$^{-6}$, but in some higher levels a markedly different value is derived, which may, however, arise through the effects of Fermi resonance.