The lithium intercalation into the layered dichalcogenide 3R-WS2 has been investigated by electrochemical reduction and by chemical reaction in n-butyl lithium solution. Essential results are (a) a charge transfer of nearly 0.6e(-)/W in LixWS2, (b) a small increase of the c-axis parameter of about 0.6%, and (c) a high mobility of the Li+-ions. The chemical diffusion coefficient of Li+-ions is estimated to be 8 x 10(-9) cm(2) s(-1) in the composition range 0 less than or equal to x less than or equal to 0.25. The appearance of a structural transformation from 3R-WS2 to 2H-LixWS2 is interpreted on grounds of instabilities in the interlayer structure.