Abstract: To liquid-treat filamentary materials, such as threads, yarns and textiles, a hollow package of the filamentary material is confined within a vessel so that the package subdivides the interior of the vessel in an inner and a separate outer chamber which communicate with one another only through the thickness of the filamentary material. A foamable treating liquid in unfoamed state is admitted into one of the chambers and is pressed into the filamentary material in order to wet the filamentary material with the liquid. The still unfoamed liquid is then discharged from said one chamber and a pressurized gaseous fluid is forced through the package in order to evenly distribute the wetting liquid throughout the package and to foam it at the same time.