Self-assembled monolayers (SAMs) of omega-(4'-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)(2)(CH2)(n)SH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Preparation at elevated temperature results in highly ordered layers with large domains. Whereas the (2root3 x root3) structure of the BP3 SAMs is similar to that reported previously for other aromatic thiols SAMs made from BP4 exhibit a very different structure. A (5root3 x 3)rect unit cell containing 8 molecules is found which corresponds to a packing density reduced by 25% compared to that of BP3. The odd-even effect observed in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenylbased SAMs.