The high speed spectrometer described here allows the measurement of an extinction or reflection spectrum within the range of 2300–6000 Å (respectively, 3500–7000 Å) or any desired sections of same. The spectra can be shown individually or in superposition (4, 8, or 16 spectra) on the screen of an oscilloscope, the x axis expressing the wavelength and the y axis the extinction. Under suitable conditions, the constancy of the zero line is better than 0.003 o.d. and allows the readings of differences in optical density less than 0.001 o.d. Examples of applications show that the apparatus is particularly suitable for following reactions in which several components simultaneously vary their absorption.