Measurements of ${\mathrm {B}} ^{{\mathrm {s}}2}(5840)^0 $ and ${\mathrm {B}} {{\mathrm {s}}1}(5830)^0 $ mesons are performed using a data sample of proton-proton collisions corresponding to an integrated luminosity of , collected with the CMS detector at the LHC at a centre-of-mass energy of $8,\text {TeV} $ . The analysis studies P-wave ${\mathrm {B}} ^0{\mathrm {s}} $ meson decays into ${\mathrm {B}} ^{()+} \mathrm {K} ^- $ and ${\mathrm {B}} ^{()0} \mathrm {K} ^0_{\mathrm {S}} $ , where the ${\mathrm {B}} ^+ $ and ${\mathrm {B}} ^0 $ mesons are identified using the decays ${\mathrm {B}} ^+ \rightarrow {\mathrm {J}}/\psi \mathrm {K} ^+ $ and ${\mathrm {B}} ^0 \rightarrow {\mathrm {J}}/\psi \mathrm {K} ^{}(892)^0 $ . The masses of the P-wave ${\mathrm {B}} ^0_{\mathrm {s}} $ meson states are measured and the natural width of the ${\mathrm {B}} ^{{\mathrm {s}}2}(5840)^0 $ state is determined. The first measurement of the mass difference between the charged and neutral ${\mathrm {B}} ^{} $ mesons is also presented. The ${\mathrm {B}} ^{{\mathrm {s}}2}(5840)^0 $ decay to ${\mathrm {B}} ^0 \mathrm {K} ^0_{\mathrm {S}} $ is observed, together with a measurement of its branching fraction relative to the ${\mathrm {B}} ^{{\mathrm {s}}2}(5840)^0 \rightarrow {\mathrm {B}} ^+ \mathrm {K} ^- $ decay.