The effect of high pressure up to 2400 bar on the UV–Vis spectra of three azo dyes, Acid Red 266 (AR), Benzopurpurine 4B (BP) and Congo Red (CR), in ethane-1,2-diol and water was investigated. The pressure dependences of the wavenumbers at maximum absorbance, νmax(p), recorded in both solvents, were determined and from those the corresponding gradients, dνmax/dp. Additionally, derivative spectroscopy was applied and essentially two bands could be resolved in the wavelength range 400–750 nm. Their influence on both the intensities, Amax(p), and the pressure-induced shifts of the peak positions at maximum absorbance is discussed. The wavenumbers of the maxima of the resolved bands (recorded in ethane-1,2-diol) show roughly the same pressure dependence when compared with each other, but exhibit significant differences for water as solvent. This different behaviour is explained by dyestuff aggregation in water, which is suppressed when ethane-1,2-diol is used. A brief comparison of the pressure behaviour of the azo dyes under investigation with respect to their aggregate structures with that of some cyanine dyes studied previously is included.