We present dynamically- determined rotation- curve mass decompositions of 30 spiral galaxies, which were carried out to test the maximum- disk hypothesis and to quantify properties of their dark- matter halos. We used measured vertical velocity dispersions of the disk stars to calculate dynamical mass surface densities (Sigma(dyn)). By subtracting our observed atomic and inferred molecular gas mass surface densities from Sdyn, we derived the stellar mass surface densities (S*), and thus have absolute measurements of all dominant baryonic components of the galaxies. Using K- band surface brightness profiles (IK), we calculated the K- band mass- to- light ratio of the stellar disks (gamma* = S*/IK) and adopted the radial mean (gamma*) for each galaxy to extrapolate S* beyond the outermost kinematic measurement. The derived.* of individual galaxies are consistent with all galaxies in the sample having equal.*. We find a sample average and scatter of Sigma.* Sigma = 0.31 +/- 0.07. Rotation curves of the baryonic components were calculated from their deprojected mass surface densities. These were used with circular- speed measurements to derive the structural parameters of the dark- matter halos, modeled as either a pseudo- isothermal sphere (pISO) or a Navarro- Frenk- White (NFW) halo. In addition to our dynamically determined mass decompositions, we also performed alternative rotation- curve decompositions by adopting the traditional maximumdisk hypothesis. However, the galaxies in our sample are submaximal, such that at 2.2 disk scale lengths (hR) the ratios between the baryonic and total rotation curves (F 2.2hR b) are less than 0.75. We find this ratio to be nearly constant between 1- 6hR within individual galaxies. We find a sample average and scatter of Sigma F 2.2hR b Sigma = 0.57 +/- 0.07, with trends of larger F 2.2hR b for more luminous and higher- surface- brightness galaxies. To enforce these being maximal, we need to scale.* by a factor 3.6 on average. In general, the dark- matter rotation curves are marginally better fit by a pISO than by an NFWhalo. For the nominal-.* (submaximal) case, we find that the derived NFW- halo parameters have values consistent with.CDM N- body simulations, suggesting that the baryonic matter in our sample of galaxies has only had a minor effect on the dark- matter distribution. In contrast, maximum-.* decompositions yield halo- concentration parameters that are too low compared to the.CDM simulations.