The development of molecular and cellular magnetic resonance imaging (MRI) procedures has always represented a challenge because of the fact that conventional MRI contrast agents are not directly detected in vivo; in proton MRI (e.g., with the nucleus ¹ H), their local concentration is measured through the effect they exert on the signal of hydrogen protons present in their immediate vicinity. Because the contrast effects generated by conventional MRI probes superpose to and can often impede the anatomical information contained in ¹ H MRI images, new probes based on a nucleus other than ¹ H, are being developed. In this study, we report on the development of fluorinated mesoporous silica nanoparticles (MSNs), which could represent an interesting dual probe that allows two MRI modes: ¹ H for high-resolution anatomical information and ¹⁹ F for the detection of MSNs used as drug delivery agents. MSNs were synthesized and covalently functionalized either with fluorosilane (FMSNs) or polyfluorosiloxane (polyFMSNs) to enable their detection in ¹⁹ F MRI. Then, gadolinium chelates were grafted on the particles to enhance their detectability in ¹ H MRI. The physicochemical, textural, and relaxometric properties ( ¹ H and ¹⁹ F relaxation times) of the nanoparticles were measured and compared. The ¹⁹ F relaxation properties were found to be dependent on the concentration of fluorine; they were also highly sensitive to the presence of gadolinium. The shortest relaxation times were obtained with polyFMSNs. At clinical magnetic field strengths, high ¹ H relaxivities and low relaxometric ratios (r ₂ /r ₁ = 1.45; 2.2 for nanoparticles entrapped in hydrogel) were found for both nanoparticle systems. Finally, the visibility of both systems was confirmed in ¹ H, and the detectability of polyFMSNs was confirmed in ¹⁹ F MRI. This physicochemical and relaxometric study opens the door to the applications of fluorinated silica nanoparticles as theranostic materials allowing dual MRI ( ¹ H and ¹⁹ F).