This study investigates the mechanism of tonal contraction when a disyllabic unit is merged into a monosyllable at fast speech rate in Taiwan Mandarin. Various degrees of contraction of bi-tonal sequences were elicited by manipulating speech rates. Functional Data Analysis was performed to compare trajectories of F0 and velocity in the contracted and non-contracted syllables. Preliminary results show that speakers always make an effort to produce the original tones, even in cases of extreme degrees of reduction. This finding militates against phonology-based accounts like the Edge-in model, according to which contraction is a process of deleting adjacent tonemes while leaving the non-adjacent tonemes intact.