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Stable functionalization of semiconductor surfaces is a prerequisite for all-semiconductor mid-IR biophotonics. This work demonstrates the adsorption of phosphonic acids on oxygen plasma activated GaSb and InAsSb surfaces. X-ray photoelectron spectroscopy shows that oxygen plasma treatment, used to activate the investigated III–V surfaces, increases the surface oxide layer beyond its native oxide thickness. Phosphonates with different terminal groups, either ethylene glycol or fluorinate carbon terminated groups, allow to modify the hydrophobicity of the surfaces and to protect the surfaces by an anti-fouling cover layer. Infrared spectroscopy indicates partial deprotonation of the phosphonic acid and thus phosphonate bonding to the surfaces. Adsorption of phosphonates on an all-semiconductor mid-IR plasmonic grating structure is detected by a shift and by a shape modulation of the plasmonic resonance peak. Compared with molecule adsorption on flat mirror-like layers a ten-fold signal enhancement is found. The adsorbed molecules are stable upon baking at 120° C, ultrasonic cleaning with organic solvents and storage for several weeks at ambient conditions. These results show that stable functionalization of InAsSb and GaSb surfaces by phosphonate monolayers is possible. All-semiconductor enhanced plasmonic sensing in the mid-IR was demonstrated.