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For the study of high-temperature gas dynamics, shock-tube techniques have been developed earlier to produce shock waves strong enough to heat gases to high and accurately known enthalpy (for argon up to 18 000°K or 40 percent ionization at equilibrium). This paper reports a study of the visible radiation from the highly luminous argon following strong shock waves. Preliminary spectrograms showed a strong continuum and that the prominent argon lines were broadened and shifted to the red. Correlation of the frequency shifts with theoretical treatments permitted an evaluation of the ion density in the gas. Development of a drum camera spectrograph (film speed 700 ft/sec) in which time effects could be resolved to about 1 μsec indicated that equilibrium ion density was reached rapidly, and provided a rough measurement of the rate of decline of ion density due to cooling. Absolute photoelectric spectrophotometric measurements of the continuum radiation were made and correlated with theoretical expectations. Confirmation of the expected variation of continuum intensity with wavelength and temperature was obtained and an undetermined factor on the theoretical intensity was evaluated. Determinations of the cooling rate of the high-temperature argon from time variation of the continuum intensity, the line shift, and the electrical conductivity (by others) are in agreement and show that continuum radiation was the dominant heat loss.