Due to the increasing importance of organic-based electronic and optical devices, there is much interest in developing attachment chemistries to join organic structures with inorganic semiconductor substrates. In this presentation, the use of unsaturated hydrocarbons to form the initial organic layer by direct bonding to silicon and germanium surfaces in vacuum will be discussed. A series of unsaturated molecules, both cyclic and non-cyclic, have been investigated using various methods including multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and temperature-programmed reaction/desorption (TPR/D). Conjugated dienes such as butadiene can undergo Diels-Alder-like [4+2] cycloaddition reactions on both Si(100)-2x1 and Ge(100)-2x1 surfaces, whereas non-conjugated dienes and alkenes react by direct [2+2] addition. The relative reactivity of Si(100)-2x1 and Ge(100)-2x1 toward unsaturated molecules will be compared. An overview of the bonding and thermal reactivity of the different compounds will be presented, and the potential for these types of cycloaddition reactions in future applications will be discussed.