The Envelope Impedance (EI) concept was used to investigate the thermal effects on the stability of Piezoelectric Transformers (PT) based ballasts for Cold Cathode Fluorescent Lamps (CCFL) systems. The envelope analysis of the CCFL, which was verified experimentally, revealed that the magnitude of the negative EI decreases with temperature. This might cause the system to become unstable at low temperatures due to the fact that the loop gain of the PT-CCFL system might encircle the (-1, 0) point in the Nyquist plan. In such cases, the PT-CCFL system may enter a quasistable oscillatory operation mode in which the current is AM modulated by a low frequency parasitic signal. This phenomenon is analyzed and explained by considering the temperature effect on the EI of the CCFL. The results of this work provide an insight into the stability issue of the PT-CCFL system, and could help in finding remedies to the instability.