The authors present experimental results obtained by far-infrared reflection spectroscopy in the investigation of the optical phonons in ultrathin-layer (GaAs)_n(AlAs)_m superlattices (UTSLs). It was found that the frequencies of confined optical phonons in UTSLs are much closer to the frequency of the corresponding bulk phonon at the centre of the Brillouin zone than in SLs with sufficiently thick layers. The influence of the barrier thickness was also studied.