The property of expansion of cassava sour starch is very valued allowing the production of expanded gluten-free biscuits without need of extrusion, baking powder or yeast addition. Lactic fermentation and sun-drying are involved in the modification of cassava starch and are linked with the baking ability. Photo-oxidation was suggested as possibly responsible for chemical alterations of the starch macromolecules and in the present work cassava starch oxidation was considered. Potassium permanganate was employed for starch oxidation followed by lactic (LAC) or citric (CIT) acid treatment. Native cassava starch (NAT) as well as lactic acid treated and oven (LACOV) or sun-dried (LACSUN) samples were considered for comparison. One sample of commercial cassava sour starch (SOUR) was also analyzed. The results showed that both chemically (LAC and CIT) and photo-chemically (LACSUN and SOUR) modified samples presented baking property, but not NAT and LACOV. The carboxyl content was higher for the chemically oxidized samples indicating that they were more extensively modified. The FTIR spectroscopy data of these and some other samples resulted in a separation by their spectra, after being studied by principal component analysis (PCA). The presence of carboxylate groups (1600 cm^-1) was essential for differentiating the samples. By using partial least squares regression (PLS) on mean normalized data, it was possible to predict the expansion of the samples, that was positively related with carboxylate band (1600 cm^-1) and negatively related with another band at around 1060 cm^-1, that was assumed to be due to a degradative oxidation taking place at C-O bond (carbon 1 and oxygen 5) of the cyclic part of glucose.