Sourdough (SD) is a fermented dough used traditionally as a natural leavening agent in baked goods. The aim of this work was to microbiologically and biochemically characterize Type 1-SD prepared with agro-industrial by-products such as soybean extruded--expelled meal (SD-SEE) and rice bran (SD-RB). SD-SEE and SD-RB were prepared by mixing tap water and flour in ratios of 2.5:1 and 1.5:1, with dough yields of 350 and 250, respectively. Fermentations were carried out daily at 25°C for 8 h, after a 24 h initial fermentation, with refrigeration at 4°C for 16 h between each fermentation. The backslopping procedure used 25% (w/w) of the previous day's SD as inoculum for the new flour mixture. Enterobacteriaceae, lactic acid bacteria (LAB), and yeasts were enumerated on specific agar media under controlled temperature conditions; pH and total titratable acidity were determined; and carbohydrates, lactic, and acetic acid concentrations were quantified using HPLC. After 10 days of fermentation, a significant decrease in pH was observed, from initial values of 6.7 (SD-SEE) and 6.31 (SD-RB) to 4.5 and 4.2, respectively. Furthermore, a notable increase in lactic and acetic acid concentrations (mM) was recorded in both SDs, underlining the significant microbial activity during the fermentation process. Cell densities of presumptive LAB (log CFU g^-1) and yeasts (log CFU g^-1) increased significantly, while a decrease in carbohydrate content (mM) was observed, indicating their utilization by the fermenting microorganisms. On the other hand, the culture-dependent method did not detect Enterobacteriaceae counts (log CFU g^-1) due to the acidic environment of SD. Selected LAB and yeast isolates from the final SD samples, identified by the culture-dependent method, revealed Pediococcus pentosaceus, Levilactobacillus brevis, and Saccharomyces cerevisiae as the dominant species. These findings highlight the promising use of industrial by-products in GF-SD, underscoring the importance of continuing to explore and optimize these processes to enhance the nutritional and sensory quality of the final bakery products.