In this paper, we describe two characterization methodologies for dual-band HF/UHF RFIC chips and antennas with single and differential-ended port configurations. Test fixtures and de-embedding techniques are used to achieve an accurate and robust impedance characterization of an RFID chip and a corresponding antenna. The multiline TRL (thru-reflect-line) de-embedding technique is employed to eliminate the unwanted behavior of the test fixtures from the measured response. In addition, a new mode transition from differential ground-less coupled line to single-ended line is introduced to characterizing the input impedance of the UHF antenna. The simulation and measurement results are in good agreement and demonstrate the accuracy of the proposed method.