In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6-x system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1-xWx)O6-x samples with 0 ≤ x ≤ 1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN ≈ 50 K for x ≥ 0.6. In samples with 0.7 ≤ x ≤ 0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistance effect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMo6-w. The coexistence of the two types of magnetoresistance effect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para-to ferrimagnetic transition at Tc.