An understanding of the effect of partial premixing on flame characteristics is important to improve the design of many practical applications. In this study, a numerical investigation is carried out to examine the effect of time-dependent partial premixing in counterflow diffusion flames. The objective is to help understand the effect of turbulent fluctuations on a flamelet embedded in the how held. Premixing of both air with the fuel stream and of fuel with the air stream is considered. The equivalence ratio of the partially premixed reactants is varied as a function of time. As expected, unsteady partial premixing results in a fluctuating double-flame configuration: a premixed flame and a diffusion flame. The maximum flame temperature, heat release rate and radiative heat loss are used to describe the flame response to fluctuations in partial premixing. The increase in partial premixing raises the diffusion flame temperature. This increase in temperature is greater at higher strain rates due to closer proximity of the premixed flame. The flame dynamics to changes in partial premixing is influenced by similar physical processes as those responsible for flame dynamics of pure diffusion flames subjected to unsteady effects. At low fluctuation frequencies, the flame responds quasi-steadily to changes in partial premixing. However, the amplitude of the flame response decreases gradually with an increase in the fluctuation frequency. At large frequencies, the flame is effectively insensitive to changes in partial premixing. A modified Strouhal number (based on the ratio of imposed fluctuation frequency and strain rate) is defined to predict the flame response to imposed fluctuation in partial premixing and to identify a regime where transient effects are important.