Microbubbles are a well-established intravascular ultrasound contrast agent. There is increasing interest in MRI-guided microbubble-mediated focused ultrasound treatments such as thermal surgery. MRI magnitude has a non-linear and non-local dependence on microbubble size and volume fraction, making it unsuitable for estimating microbubble concentrations. Quantitative Susceptibility Mapping (QSM) is a strong candidate for tracking microbubble concentration, destruction and clearance because the susceptibility depends linearly on the volumetric bubble concentration. Here, we show the first QSM of microbubbles in a phantom and observe that the measured susceptibility has a high SNR and is directly proportional to the microbubble volumetric concentration.