Four species of gonyleptid harvestmen, Acanthogonyleptes pulcher, Gonyleptes saprophilus (Gonyleptinae), Sodreana barbiellini, and Sodreana leprevosti (Sodreaninae), were examined by GC-MS and ¹³H NMR. All of these species release vinyl ketones, and three of them produce the corresponding pyranyl ketones, which are presumed hetero-Diels-Alder (HDA) dimers. The vinyl ketones 5-methyl-1-hexen-3-one, rac-4-methyl-1-hexen-3-one, and (S)-4-methyl-1-hexen-3-one were synthesized. Natural 4-methyl-1-hexen-3-one is present as a single stereoisomer and has the R-configuration. Vinyl ketone dimers (HDA dimers) were also observed in the scent gland exudate and characterized by HRMS, ¹³C NMR, and ¹H NMR chemical shifts of the pyranyl moiety.