New measurements are reported for the seismic velocities in upper oceanic crust (layer 2A) at off-axis sites along a track of 34 ¿N in the Pacific where the sediments were thin (<33m) and the crustal age increased from 42 to 63 m.y. The data were obtained in experiments designed to measure the reflection coefficient at each site using a horizontal line array. The reflection coefficient versus angle information was inverted for the seismic velocities using a nonlinear inversion technique based on a simulated annealing search algorithm. The p wave velocity increased from 2.7 to 3.1 km/s along the track, and the shear velocity increased from about 1.0 to 1.6 km/s. These results suggest that the aging process controlled by passive hydrothermal circulation is continuing at these sites. ¿ American Geophysical Union 1994