This contribution develops an analytical procedure for the proximate analysis of algal biomass, using Spirulina platensis and Chlorella vulgaris as model algal samples. The procedure consists of three steps. First, thermogravimetric analyzer (TGA)-based method designed from ASTM E870-82 was employed for the proximate analysis of the two algae, at 950 °C in argon for the determination of volatile matter (VM) and fixed carbon (FC) and at 600 °C in air for estimating the ash content. Visual inspection of the resulting solid residues suggested the presence of black unburnt carbon, indicating the inapplicability of the method. Nevertheless, the contents of moisture, VM, and the sum of FC and ash in the two algae were determined in this step. In the second step, Spirulina platensis and Chlorella vulgaris were directly oxidized at 600 °C in air, in the same TGA as above, to avoid or minimize the effect of stable quaternary nitrogen formed during pyrolysis at 950 °C in the first step. While Spirulina platensis was successfully ashed in this step, the solid residue of Chlorella vulgaris was virtually black. This necessitated the third step, in which Chlorella vulgaris was ashed in the air at 600 °C in a tubular reactor, with the aid of hydrogen peroxide. Nearly complete oxidation of carbonaceous material in Chlorella vulgaris was achieved when the holding time at 600 °C was 4 h. Based on the proposed analytical procedure, the contents of VM, FC, and ash are 82.3, 10.4, and 7.3 wt %, respectively, for Spirulina platensis and 84.3, 10.4, and 5.3 wt % for Chlorella vulgaris, expressed on a dry basis.