I. A photographic spectroohotometric method has been used to estimate the position of the band-maximum of chlorophyll (a + b) in different media and in the living leaf. A description of the methods and instruments is given. II. The pigment was prepared after the method of W i 11- statter and Stoll. III. The pigment follows Beer and Lambert’s law over a considerable range of concentrations in all the media studied. IV. The effect of photooxidation is studied. Spectroscopic measurements of the position of the first (main) absorption band are not influenced by photooxidation. An approximate absorption spectrum of the photooxidised product is given. V. The shifting of the band maxima In organic media follows, in general, Kundt’s ’’law”. The specific extinction is not altered in these media. The band moves ”bodily” through the spectrum, excepted in methyl, alcohol. In this medium the absorption band shows a kind of wing to the longer wavelengths. VI. A special method is described to prepare colloidal solutions of chlorophyll. The position of the band-maximum of the pigment in the colloidal $tate is dependent upon the degree of dispersion of the pigment. Addition of bivalent ions to solutions of colloidal chlorophyll decreases the dispersion. With decreasing dispersion of the pigment, the band maximum shifts towards the longer wavelengths. VII. Dry chlorophyll has the same absorption maximum as the living plastid, but the pigment in this state is non-fluorescent. VIII. The system chlorophyll-lecithin-water is strongly fluorescent but shows a shift of too A to the shorter wavelengths. IX. The band maxima of several leaves are found to be practically identical. The maximum is found between 6800—6810 A. When leaves are ground the maximum shows no appreciable shift. On boiling the leaves, the maximum shifts to the shorter wavelengths. The maximum of the band of boiled leaves is found practically on the same place as the maximum of a system chlorophyll-lecithin-water. X. Plastids photographed in red light show that the absorbing complex is distributed inhomogeneously in the stroma of the chloroplastid. Some calculations have been made on the amount of chlorophyll in one single chloroplastid. The pigment cannot be present in one monomolecular layer around the surface of the plastid. XI. An attempt has been made to explain the structure of the plastid. Based on our experiments and the work of Noack (jo) and Mestre (45) the tentative conclusion Is reached that the ’’phyllochlorin” complex consists of a protein complex, lecithinoids and the four well known pigments.