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Methylacetylene and methyldiacetylene are the first members of the methylpolyynes series, CH₃C_2nH (n = 1, 2). Their astrophysical relevance has motivated this spectroscopic study. The aim of this investigation is to provide very accurate rest frequencies, as well as to improve the spectroscopic parameters available in the literature for these molecules. The Lamb-dip technique was exploited in order to record the rotational spectra of CH₃CCH and CH₃CCCCH at sub-Doppler resolution in the millimeter- and submillimeter-wave frequency ranges. In addition, for CH₃CCH rotational transitions in the THz region were recorded at Doppler resolution. We report the most accurate ground-state rotational parameters known at the moment for the main isotopic species of methylacetylene, as well as of methyldiacetylene. Since both molecules are known to be of astrophysical relevance, we are confident that the improvement in the accuracy of the ground-state rotational parameters as well as of the rest frequencies of rotational transitions could be useful for future observations purposes.