The solid state complexes, solvatochromic and acidochromic behaviour of four merocyanine-type dyes derived from barbituric and thiobarbituric acid and their use as solvatochromic probe molecules for coloured surfaces are described. The dyes were obtained by condensation reaction of barbituric, N-methylbarbituric, N,N′-dimethylbarbituric and thiobarbituric acid with 4-N,N-dimethylaminocinnamaldehyde. The dyes were characterised by means of liquid and solid state NMR techniques (¹H MAS NMR, ¹H–¹H DQ MAS NMR, ¹³C CPMAS NMR), single-crystal X-ray analysis, UV/vis and IR measurements. The solvatochromism has been investigated in 43 solvents and interpreted in terms of Kamlet–Taft parameters α, β, and π*. Altogether, the solvatochromic properties of these four dyes were determined mainly by the hydrogen bond donating (HBD) ability and the polarisability/dipolarity of a solvent. The interactions of the dyes with oxidic and metal surfaces were studied. Their perichromic behaviour was compared with that of established solvatochromic dyes for the determination of α, β, and π*, namely dicyano-bis-(1,10-phenanthroline)-iron(II)-complex (1), 3-(4-amino-3-methylphenyl)-7-phenyl-benzo-[1,2-b:4,5-b′]-difuran-2,6-dione (2) and 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Δ³-thiazoline (3). The barbituric acid dyes were used to probe the surface polarity of coloured oxides like tungsten(VI) oxide and iron(III) oxide. The interactions between the dyes and metals like zinc and aluminium were also investigated.