The DØ Collaboration has studied for the first time the properties of hadron-collider jets reconstructed with a successive-combination algorithm based on relative transverse momenta (κ⊥) of energy clusters. Using the standard value D = 1.0 of the jet-separation parameter in the κ⊥ algorithm, we find that the pT of such jets is higher than the ET of matched jets reconstructed with cones of radius R = 0.7, by about 5 (8) GeV at pT ≈90 (240) GeV. To examine internal jet structure, the κ⊥ algorithm is applied within D = 0.5 jets to resolve any subjets. The multiplicity of subjets in jet samples at √s = 1800 GeV and 630 GeV is extracted separately for gluons (Mg) and quarks (Mq), and the ratio of average subjet multiplicities in gluon and quark jets is measured as (〈Mg〉-1)/(〈Mq〉-1) = 1.84±0.15 (stat)±0.18 0.22 (syst). This ratio is in agreement with the expectations from the HERWIG Monte Carlo event generator and a resummation calculation, and with observations in e+e- annihilations, and is close to the naive prediction for the ratio of color charges of CA /CF = 9/4 = 2.25.