Self-assembled monolayers (SAMs) of two omega-(4'-methylbiphenyl-4-yl) alkanethiols (CH3(C6H4)(2)-(CH2)(n) SH, BPn, n = 4, 6) on Au(111) substrates, prepared from solution at room temperature and subsequently annealed at temperatures up to 493 K under a nitrogen atmosphere, were studied using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BP n SAMs with n = odd, for which only one phase is observed, the even-numbered BP n SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in density and stability. Upon annealing, BP4 and BP6 transform into a beta-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diameter. Exchange experiments, monitored by contact angle measurement, reveal that the, beta-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even-and odd-numbered BP n SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.