While the structures of self-assembled monolayers (SAMs) of alkanethiols on Au(111) are extensively studied and well-known, new structures and complex phase behavior have been progressively discovered when coverage of these layers falls below saturation. Structures and phase transitions of annealed decanethiol monolayers on Au(111) surfaces were systematically investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Rich structures were revealed as a result of annealing in UHV. At temperatures below 345 K, no significant changes in coverage were observed, although the size of two-dimensional crystalline c(4root3 x 2root3)R30degrees domains increases as annealing progresses. A two-dimensional melting occurs at 345 +/- 5 K and was captured in situ from time-dependent STM studies. Above 400 K, significant desorption takes place. In the temperature range of 345-400 K, within which desorption progresses to gradually decrease the surface coverage, a variety of striped phases have been observed, each having distinct molecular-level packing and unit cells. Well-known striped phases have been confirmed: (p x A), with p values (integer or half-integer multiples of the Au(111) periodicity) of 7.5, 9, and 11. In addition, new structures such as mixed striped phases and mesh-like structures are revealed, which are often found to coexist with the regions of pure striped phases. The systematic investigations of the structural and phase evolution shed light on the SAM desorption process at the molecular level.