This paper describes the fabrication and characterization of photopolymerizable alkylphosphonate self-assembled monolayers (SAMs) on group-III nitride substrates including GaN and AlxGa1-xN (AlGaN; x = 0.2 and 0.25). Contact angle goniometry, visible absorption spectroscopy, and atomic force microscopy were used to assess the formation, desorption, and photopolymerization of SAMs of diacetylenic alkylphosphonicl acids (CH3(CH2)(n) -C equivalent to C-C equivalent to C- (CH2)(m)PO(OH)(2); (m, n) = (3, 11), (6, 8), and (9, 5)). As with GaN substrates (Ito, T.; Forman, S. M.; Cao, C.; Li, E.; Eddy, C. R., Jr.; Mastro, M. A.; Holm, R. T.; Henry, R. L.; Hohn, K.; Edgar, J. H. Langmuir 2008, 24, 6630-6635), alkylphosphonic acids formed SAMs on UV/O-3-treated AlGaN substrates from their toluene solutions in contrast to other primary substituted hydrocarbons with a terminal -COOH, -NH2, -OH, or -SH group, Diacetylenic alkylphosphonate SAMs on group-Ill nitrides could be polymerized by UV irradiation (254 nm), as indicated by the appearance of a visible absorption band around 640 nm and also by their significantly reduced desorption from the surface in a 0.1 M aqueous NaOH solution. A longer UV irradiation time was required to maximize the photopolymerization of a SAM having a diacetylene group close to the terminal phosphonatc moiety, probably because of the hindrance of the topochemical polymerization due to the limited flexibility of the cross-linking moieties on an atomically rough substrate surface.