A homologous series of para-substituted phenyloctadecyl ethers (X-POEs, X = H, Cl, Br, and I) was prepared using Williamson＇s ether synthesis. The structure and purity of the ethers were confirmed using H-1 NMR and GC/MS. Scanning tunneling microscopy (STM) images were acquired from monolayers of the ethers which formed at the surface of highly oriented pyrolytic graphite from a 2.5 wt % solution in phenylhexane. For all four ethers, the monolayers displayed a contrast which varied as a function of tip-sample bias. A comparison of STM images of the adsorbed ether molecules with electron density contours calculated using HyperChem suggested a bias-dependent participation to tunneling of individual bonding molecular orbitals (MOs). For example, at biases of -0.26 to -0.70 V, STM images of I-POE resembled the highest occupied molecular orbital (HOMO) exhibiting two bright spots for the pair of lobes of the phenyl ring and one bright spot for the halogen atom. At higher biases, contrast was observed for the phenyl ring alone (-0.8 to -1.2 V) and for the alkyl tail (-1.0 to -1.8 V) which was similar to that of the HOMO-1 and HOMO-4 contours, respectively. Owing to the measurement of an enhanced tunneling current simultaneous with the acquisition of atomically resolved bias-dependent STM images of the X-POE adsorbates, a resonance tunneling mechanism between the tip and substrate via MOs of molecular adsorbates adjacent to and including the HOMO is proposed.