A method of measuring a percent solids content (PSC) of solids by mass in a slurry catalyst composition having solids and a suspension liquid for the solids, wherein the solids of the slurry catalyst composition include a catalyst for use with a polymerizable feedstock in a polymerization reactor to produce a polymer, the method comprising: preparing a test sample of the slurry catalyst composition; obtaining a first time domain (TD) 1 H-nuclear magnetic resonance (NMR) spectrum using a time domain (TD)-NMR spectrometer and the test sample of the slurry catalyst composition used in producing the polymer in the polymerization reactor, where the first TD 1 H-NMR spectrum is measured in the TD-NMR spectrometer at a temperature in the range of 10 °C to 70 °C; determining a value of a voltage signal (a) from the first TD 1 H-NMR spectrum that represents the slurry catalyst composition; preparing a neat sample of the suspension liquid for the solids of the slurry catalyst composition; obtaining a second TD 1 H-NMR spectrum using the TD-NMR spectrometer for the neat sample of the suspension liquid for the solids of the slurry catalyst composition, where the second TD 1 H-NMR spectrum is measured in the TD-NMR spectrometer at the same temperature used in obtaining the first TD 1 H-NMR spectrum; determining a value of a voltage signal (b) from the second TD 1 H-NMR spectrum that represents the suspension liquid for the solids of the slurry catalyst composition; and determining the percent solids content (PSC) of solids by mass in a slurry catalyst composition with Equation I: PSC = 1 − a / b × d b / d a × 100 % where x represents mathematical multiplication, a and b are as described above, d b is a density of the suspension liquid for the solids of the slurry catalyst composition and d a is a density of the slurry catalyst composition used in producing the polymer in the polymerization reactor, where the densities d b and d a are both measured at the temperature used in obtaining the first TD 1 H-NMR spectrum. preparing a test sample of the slurry catalyst composition; obtaining a first time domain (TD) 1 H-nuclear magnetic resonance (NMR) spectrum using a time domain (TD)-NMR spectrometer and the test sample of the slurry catalyst composition used in producing the polymer in the polymerization reactor, where the first TD 1 H-NMR spectrum is measured in the TD-NMR spectrometer at a temperature in the range of 10 °C to 70 °C; determining a value of a voltage signal (a) from the first TD 1 H-NMR spectrum that represents the slurry catalyst composition; preparing a neat sample of the suspension liquid for the solids of the slurry catalyst composition; obtaining a second TD 1 H-NMR spectrum using the TD-NMR spectrometer for the neat sample of the suspension liquid for the solids of the slurry catalyst composition, where the second TD 1 H-NMR spectrum is measured in the TD-NMR spectrometer at the same temperature used in obtaining the first TD 1 H-NMR spectrum; determining a value of a voltage signal (b) from the second TD 1 H-NMR spectrum that represents the suspension liquid for the solids of the slurry catalyst composition; and determining the percent solids content (PSC) of solids by mass in a slurry catalyst composition with Equation I: PSC = 1 − a / b × d b / d a × 100 % where x represents mathematical multiplication, a and b are as described above, d b is a density of the suspension liquid for the solids of the slurry catalyst composition and d a is a density of the slurry catalyst composition used in producing the polymer in the polymerization reactor, where the densities d b and d a are both measured at the temperature used in obtaining the first TD 1 H-NMR spectrum. <EOS>