Diffusion coefficients, D, have been measured at five temperatures for melts of 14 samples of polybutadiene with varying vinyl content, f(v), covering a broad range. Seven of the samples have a molecular weight, M, near 1300, and the other seven have M near 3400. The experiments show that D tends to decrease with increasing f(v). This trend in the experiments can be rationalized by two seemingly different approaches. One successful approach utilizes free volume theory. The other successful approach is a Monte Carlo simulation that employs different jump rates for beads representing the two types of butadiene units. The two methods may not be as different as they appear initially, because they both incorporate, directly or indirectly, the fact that the glass transition temperature moves closer to the temperature used for measurement of D as f(v) increases.