In the food industry, acetic acid is a compound often used as a preservative or additive, but it is also produced during the fermentation of various food products and constitutes a key marker of food safety and quality. Although accurate, sensitive, and selective, the traditional methods for determining acetic acid are also expensive and time-consuming. In this regard, optical fiber sensors have emerged as a possible alternative for the real-time monitoring of small molecules. In this work, we propose an optical fiber sensor for the simultaneous measurement of refractive index (RI) and temperature using a balloon-shaped structure with a single sensing element. The sensor, based on cladding modal interference, was characterized by its response to RI in the range from 1.320 to 1.352 RIU, using acetic acid solutions with concentrations between 0% and 50%(v/v), and by its response to temperature between 23 and 43 ◦C. A maximum sensitivity of 170.66 nm/RIU was obtained for RI, equivalent to 110.7 pm/%(v/v) in terms of concentration, and a maximum sensitivity of - 119.2 pm/◦C was obtained for temperature.