The formation of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene derived nanostructures on hydrogen-terminated silicon(1 0 0)-2 x 1 was studied using ultra-high vacuum scanning tunneling microscopy and density functional theory modeling. We find that the steric interactions between the methyl groups on 2,3-dimethyl-1,3-butadiene and the surface hydrogen causes 2,3-dimethyl-1,3-butadiene to grow relatively long, linear structures along one side of a dimer row. However, the less sterically encumbered 1,3-butadiene grows short, less orderly, self-terminating structures that involve at least two dimer rows. These results show that steric effects can be used as a parameter in the rational design of organic nanostructures on silicon surfaces. (C) 2008 Elsevier B.V. All rights reserved.