A new method is proposed in which a chromium-based catalyst system containing chromium(III) tris (2-ethylhexanoate) (Cr(EH)(3)),2,5-dimethylpyrrole(2,5-DMP), GeCl4 and triethylaluminum (TEA) coated with solid paraffin is used for oligomerization of ethylene. GC-MS and H-1-NMR analyses show that the coated catalyst system prepared via this approach is air-insensitive so it can be stored under ambient conditions. The selectivity for 1-hexene is up to 97.91 wt.%, and only slightly decreases when the catalyst system is exposed in air for 24h; in contrast, the selectivity is only 35.6% for its non-coated counterpart. XPS reveals that an attachment of atmospheric oxygen to catalyst makes the chemical state of chromium change from Cr3+ to Cr6+. Such a change, in turn, reduces remarkably its selectivity for 1-hexene.