Calix[4]arene-thiourea and-tetraamide nakedeye receptors do not show any tendency to self-aggregation and are highly sensitive towards small monoanions; association constants in DMSO for halogenides (chloride to iodide) and HSO4-are <200 m-1. Basic anions deprotonate both receptors leading to a high and selective optical readout. Binding constants for carboxylates, fluoride, and dihydrogen phosphate are three orders of magnitude higher (∼105 m-1) in case of the tetrathiourea receptor.@en